I have to second much of the original posters opinions of the 12" PB and OSX. I also must give him a little bit of support here. Though, I don't agree with everything said.
I bought my 12" rev D in February, just after the line was refreshed (1.5GHz/512MB/Superdrive)... I went from a 1.6GHz P4 running Windows XP to my new Mac. I've been plagued by the loud fan, noisy HD and low quality screen (REALLY noticable next to even a cheap Dell laptop). The biggest problem is the dim screen with a very narrow viewing angle. Try having a friend watch a DVD with you sometime... impossible. Only one person can get their head "just right" to see the image with accurate colors and even brightness (and then only at night!).
I have encountered seveal annoying bugs in OSX... It isn't nearly as stable as my old XP system (my opinion)... I've had two major system lock ups... one incident of the keyboard being remapped to garbage and nonsense (freaked me out!). The finder has crashed several times and I've had to force quit just about every application I use regularly at least once... some things like quick time and my video player (VLC) require force quits several times a week. Don't even get me started about the beach ball. What am I doing you ask? Downloading and watching a lot of divX files of TV shows. Windows never gave me any problems playing even the lattest compression formats or very large files. But it seems OSX is behind in video playback of various codecs; there are many files I download that simply aren't playable on the Mac (yes, I have DivX, DivX doctor II, Mplayer OSX, Windows Media player, VLC, etc...). Also, the files take MUCH longer to open on my Mac than a corresponding file on my PC.. this is true of all media, types from .mp3s to photos to videos. Granted, with iTunes, the .mp3 opening time is moot once you have the file imported into the library. Also, not having good .ogg support really pissed me off initially. All media files openend nearly instentaneously on my PC... I got used to it. Now with the Mac, I must learn patience.
Applications take eons to load compared to what I was used to on XP. I don't know if it's the Intel App accelerator, Windows XP's precaching routines or what... but Opera, and Internet Explorer just launced like lightning... we're talking a small fraction of a second. Opera takes eight to ten seconds to load on OSX.

Safari has come a long way with the update to Tiger, a vast improvement in both usability and robustness (less crashing).
When opening Office documents on a PC, from a cold start with no prior use of Office, if I click on an Excel or Word file, it's open before I can say "Bill"... but on my Mac, with Office 2004, I have time to say "Bill Gates is an f***ing moron, why the heck do I have to sit here and wait all day for this..." and then it popps up. Even the smallest spreadsheet. So I get to curse at Bill more, which is an unexpected bonus to owning a Mac (I thought that routine was over when I switched).
I don't like some of the window management in OSX, the maximize window bevavior is not what I like. I find myself constantly resizing windows manually, even after resizing, not getting it right (especially when I go to click on the right scroll bar of an app and annoyingly the window behind it pops up). There should be a way to make the app exactly fill the screen... The task switching is problematic. I love the feel of expose, but it's minimally productive with many open windows (more than ten) due to the limits of the screen and the fact that it doesn't consistently place applications... each time I use it, I must hunt for the window I was looking for... and if there are multiple instances of spreadsheets that look similar, it can be confusing. So I use the "Window" file menu often.
MS Office 2004 on OSX is SLOW AS SNOT compared to Office XP. My god, it takes ten plus seconds to do a cut or copy and paste operation in some instances -- of half a line of text! I know the hardware isn't THAT slow... come on guys.
I had many problems copying my files over to the Mac from the XP box using a firewire drive. I have a lot of French language files with accent marks in them... many times I'd get beach-balled, or have the copy operation halted at an unknown place in the job. If windows has a problem with a large batch copy, it gives you useful options and lets you know exactly which file had the problem. To clarify, this isn't copy and paste... this is moving (drag-and-drop) a large directory from an external drive to the Mac HD using the finder. It was a nightmare. I even tried using the terminal with mv and had problems. (this was on 10.3.7). I did a few hours searching tech forums and couldn't find similar problems... I assumed OSX had better file name suppor than XP, but I guess not. My solution was to burn about 8GB of the PC's directory structure that contained the long filenames with accent marks to DVDs, for some reason the finder then copied them correctly. I forgot the details of the situation because it was in February and I was impatient to use my Mac for a road trip at the time. But it really soured me to my initial use of OSX (at least on my own personal machine - I've used Macs since the Mac II at work/school). It could have been the NTFS partition on the drive and the translation to OSX's filesystem that caused the agrivating problem.
Further my opinion is that the 1.5GHz G4 is no match for my three year old 1.6Ghz desktop PC. But OSX isn't nearly as responsive as Windows XP (ie, the snappy), so it may not really be the hardware... but Tiger was a good step in the right direction; a noticable improvement. Unfortunately, with Tiger, my fan seems to come on even more often... with only Safari running too! I like to leave BitTorrent running all night (and just BitTorrent, not a widget more)... and the fan is too annoying to have in my room with me. My old desktop PC was silent (I installed large Zalman heatsinks on the CPU, chipset and video card with only one large, low rpm fan for the whole system), but this Mac laptop is really loud.
Unlike the Original Poster above, I wouldn't consider switching back. I wish the 12" PB had a better video card (cooler), faster CPU, longer battery life and most importantly a better screen (the size is fine)... oh and a little thinner and lighter. I think with the switch to Intel and a little aggresiveness on Apple's part, we will see those things soon. At least compete with Sony Vaio. The GPU and CPU in the current Powerbooks are temperature elevating power hogs.
I like OSX overall... even though I think M$ is way ahead in a few key areas, probably just due to their large tech-savvy user base constantly tweaking support for various things. I do like the concept of Expose, and when I find myself on a PC, I wish it were there. Spotlight is wonderful. The Dashboard is a neat concept. I will stick with Apple for quite awhile and look forward to comming innovation. But I don't think it's a magic solution "years ahead of Windows" (maybe in some areas). For the most part it works well and looks a lot cleaner. I've been a UNIX user since the 80's and what sold me was installing X11 and running shell scripts. I dual booted Linux on my PC, but now with the Mac, I feel I don't need two OSes.
Not having to scan my system for virii and constantly remove spyware is a wonderful bonus. On that alone, I would reccommend a Mac to any of my non-gamer friends and relatives over a PC.
Mac pluses:
Expose, dashboard, spotlight
No virus scourges, no spyware (yet!)
Intelligently designed and well thought out interface
Clean, crisp interface (appearance)
iLife (especially iTunes and iMovie, very nice apps)
The PB keyboard is the best I've used on a laptop (kudos!)
I love the hinge style and speaker placement
The speakers are actually usable... though I keep my Grado's near by when I'm traveling

Nice sound for a laptop!
The PB sleeps and wakes up better/faster than any other laptop I've used!
Mac minuses:
MS Office 2004 sucks compared to Office Windows
Buggy OS (maybe less so than windows for most, but not for me)
Hardware is a ways behind the PeeCee word (couple of years from my point of view)
The Snappy... come on, use a PC, you will see. They are VERY responsive to the user... making me always impatient at my Mac
my PC felt like drive a Porsche, while the Mac feels like driving a Greyhound bus! (okay, maybe not that bad)
If anyone has suggestions on how to improve my Mac experience, let me know. Don't get me wrong, I don't "hate" my Mac... I actually love the thing, and if you stole it, I'd have to kill you. I've never had an emotional attachment to any of my PCs before (well, maybe to my original 8086 and my 486). So Apple is doing something right. But I do reserve the right to be ciritcal. I am even more critical of Windows, believe it or not... even though I think XP is very stable and robust (with a few dozen caveats

.
Right now I'm waiting for a 1GB SODIMM (Patriot, known Mac friendly)... and I plan to buy an external firewire drive, as I've filled up my 80gb HD and have a stack of DVDs in addition. When I do that, I'll format and install 10.4.1 (hopefully .2 by then)... this could solve some of my problems... as I'm still using the factory install of 10.3.7... updated to 10.3.8... then to Tiger. Something I'd NEVER do on a PC and would expect problems with (factory installs). But I was too impatient to do a clean install and started using my Mac on day one... everything is backed up on DVD, but I really don't want to go through a reinstall at the moment.
Thanks for the read... please don't flame... but you can criticise my usage habits and give feedback/tips/pointers all you like. I very well admit, perhaps I'm too used to the "Windows way" of doing things (like the OriginalPoster) and haven't fully delved into Apple's concepts.