I have the 512MB iShuffle, and it works great. Super small and light weight. It has works in two modes, either shuffle mode or in regular play mode, which is ordered how ever you set up your playlist in iTunes. Get some good "earbud" type in ear headphones. The ones that come with it suck IMHO. I have the Sony ones, they're sort of expensive, but you get excellent bass response and it cranks up pretty loud. If you're rich, go for some Shure E2 or E3s.
The iShuffle has a built-in, non-replaceable battery that charges when you plug it in via USB 2.0. The battery lasts a long time...something like 20 hours. Mine hasn't died yet, and I only charge it like once a week.