I can think of much poorer apple designs like the performa all in ones and the ugly tacked on look of the beige g3 towers to name a couple. I'm not a huge fan of the iMac g3's but considering most of them come from schools where they may have been run incessantly and abused, i'd say they've stood the test of time all right. In terms of the internal design, i would say they did a pretty clever job with the odd shaped boards etc to max out the available space. I agree, the slot load models have a slightly better look, smoother lines.
These machines weren't designed to be apple's top performers and as such their performance reflects that compared to the g3 towers. For the purpose that they were designed for, i'd say they fit the bill perfectly. If the flyback transformer goes out, it's a $ 20 part that you can re-sodder as opposed to say an ibook g3 with graphics failure that requires either a new logic board or a reflow $$$. To be honest, the only reason i'm going to keep the 333 tray loader is because a) i got it for free, B) i'm a fan of all in ones, c) will make a decent os9 retro game machine that doesn't take up piles of space.