I've only owned a Creative Zen V - I love mine. The control is slightly finicky-feeling to use (a tiny little joystick) but works fine, no problems. The software interface is very nice and pretty much indistinguishable from the iPod. The transfer from PC is a little slow compared to other players (I'm told).
It has a mic for voice recording, and an analog line-in for making digital recordings (160kbps WMAs). I used it to make digital copies of some casettes and was pretty pleased with the quality of the final product.
The "plus" version gives you radio and video capabilites. Radio is nice, but the video is pretty limited (frame rate, screen size), so I just opted for a nice deal on the non-"plus" version.
It's a flash player - mine is 2GB, they go up to 8GB, I think. 2GB easily fits about 18-20 CDs, potentially a lot more if you use low bit-rate mp3s. If you're interested in putting a huge collection on instead of switching music in and out, you'll have to check out hard-drive players instead. Can't help you there.