I recently -past couple of months- traded in my PC for the 21.5" iMac. I'm glad I did it, but sometimes don't think I got full value for money. It's not as fast as I hoped it would be. I have the standard 3.06GHz with 4GB RAM. For over £1,200 I would have hoped for better performance.
My main issues are;
Speed: It's not as quick off the mark as I would have hoped. I get the beachball quite often when running multiple programs and waking up from sleep is sluggish.
Screen: I had a yellow tint on my first one, but as far as I can tell the second one is fine.
Magic Mouse: The batteries don't go a long way, maybe 40/50 days use before changing them. Okay it's not a HUGE problem, but hey, I would hope they last longer.
Graphics: Sometimes on some games/video I get a wavey screen, like the refresh is not quick enough. I don't know if this is a faulty graphics card, or if this is just because it's an old graphics chip, but I am dissapointed it cannot handle some video/games.
Sound: The speakers are not great, however, I had some from my old PC that I could use on this, no problem. So not really a big deal for me, but if you're listening to music through it, I would recommend some external speakers.
Small gripes: No 'minimise all' like windows that I could find. This is really useful for me working of desktop documents on projects. Email cannot be reversed when deleting items, so rather than moving down the list it going up. The aesthetics of the microsoft suit are not great, not sure if this is Microsofts doing, but I don't feel at home using them as I do on a PC.
Other than these small things, everything else about this machine is perfect. I love the fact that it's free from malware (for now), that I get regular and non-intrusive updates, I don't have to click confirm 12 times just to delete a folder or move something. It's seemless with the time capsule and time machine meaning I'm always backed up. Not only that, but it saved me on a second monitor for watching DVDs in bed, as I use this, as the screen is plenty big enough.
Make the move, you wont regret it