I have a 10 gig new ipod...and windows 2k.
First off, It doesn't have ac in, but rather the firewire cable plugs into a plug base thing that connects to a wall socket. I believe I have seen car adapters for it, but never have had to use once since the battery last for 7-8 hours..and I am never in my car that long.
The software for windows is MusicMatch Jukebox..which some people like, but I think it sucks. I use ephpod instead, much better and less annoying than MMJ.
I got the 10 gig since my mp3 collection is much larger than 10 gigs, and I don't need to have it all on the player at once. So far it has been great, and the design is quite nice and simple.
I say if you have a need for having 10+ gigs of music on the go, go for it. My only complaint is that I wish it came in black or grey or something.