I just replaced the battery on my iPod Photo. The battery life was getting pretty bad and iPod batteries only last about two years anyway. I replaced the old 650 mAh battery with a 900 mAh battery. I should notice a huge difference, especially since it is larger than the old battery. It works good so far but I've only tested it while it's charging.
This will help my iPod last until the 6th gen iPod is released. If Apple follows the trend set by the iPhone and most internet rumors/fake pictures *crosses fingers*, it will hopefully look something like this:
Most rumors suggest a late 2007/early 2008 release to keep it from interfering with iPhone sales. I probably won't buy it right away anyway so mine should hold me over until I get around to buying it.