I used an iMac G5 for 6 years, and it'll do whatever you try and tell it to, rather it be working with large photoshop files or browsing flash intensive websites. What it can't do, is playback hd flash vids fluently. It will also struggle with x264/h264 encoding of videos. But asides that, it's a decently powerful computer.
Other than that, there's really no point in getting one if you can find a cheap core2duo iMac.