Aside from bug fixes and some speedup for PPC Macs, 8.1 also introduced HFS+, and other improvements. A good discussion can be found at

http://www.macintouch.com/m81.html

You may also wish to read Apple's own documentation for the 8.1 upgrade.

http://docs.info.apple.com/article.html?artnum=30345#English

Short version of the story: It's free, it can be faster, and it's stable. I always upgrade to 8.1.