iPod batteries $49 with replacement instructions.
http://www.ipodbattery.com/
The battery in an iPod powers the device for over 5 hours on a charge and is good for about 5000 recharges.
It is basically like the battery in your cell phone but being a little harder to change out. It was actually one of the selling points for me since my other players used AA batteries and they always seem to not last a long time and the connections come loose over time with me jumping up and down while working out.
At night I put my iPod in its cradle where it syncs my playlists in iTunes and charges it battery. I have 17 gigs of digitized music mostly from my CDs but most of the newer stuff is from the iTunes store. I'm hooked on being able to pickup a couple of songs a few times a week while reading the boards and listening to samples in the background as I'm surfing.
The ease of moving my playlists to and from my ipod was another strong selling point for me. Sometimes I'll just have it play random from the 17 gigs of music that is on it and if I hear a song that I really like I just need to hold down the big button in the middle of the dial for 3 seconds and it ads it to a special playlist that is called "On The Go."
I'll often quickly drag and drop and few songs that catch my interest for that day into a new playlist and then just right click update on the ipod icon and in under one minute my ipod is a duplicate of my desktop computer.
It also, at least on a Mac, mounts on the desktop so I can use it as a removable drive. Plus it syncs my address book and calender which is viewable on the ipod screen so I no longer carry a Palm pilot. My calender and address book are also synced with my Sony T610 cell phone, another product that I highly recommend.
The ipods that are being sold now are what is known as G3 ipods for 3rd generation. The earlier ones do not have 4 buttons right under the screen and do a bit less, but are still decent players. Also the earlier ones needed to be formatted Mac or PC and the newer ones don't, they just work on both platforms.