I got a new TruePower battery for my 2.5 year old 3G iPod last week. It took a grand total of 20 minutes to install, and after charging the battery, my iPod is performing better than it did the day I bought it.
I've gone entire work-days (including commute to/from the office) with about a third of the battery life left. It never went that long when the iPod was new. I no longer have to worry about my battery recharge cycle (don't forget to charge it when you get into the office, or it will be dead when you go home. don't forget to leave it in the dock all night at home, or it will be dead in the morning. etc... very liberating).
My only regret is that I don't have to buy a new iPod anymore. My old one runs so well I just can't justify a new one. I'll be able to extend the life of this iPod indefinitely, picking up a new battery every few years. That's just plain cool.
If you have a third generation iPod (the ones with the four buttons that glow), and the battery is starting to suck, run to FastMac and buy one of these batteries. For $30, you get a better-than-new iPod, while retaining the scratches and scuffs that you worked so hard to grind into your iPod...
My battery was comped by the cool folks at FastMac, but if I'd have known about the cheap and fast shipping, I would have totally bought it myself!