If you own a T-Mobile shadow, you can get a free replacement that will extend your battery life from a measly 8 hours to a decent amount. The batteries manufactured for use in the early Shadows have defects leading to a very short lifespan. If you are getting bad performance from your phone, just call T-Mobile support and they will express ship you a replacement battery that will extend life.
It’s also possible to still keep your old battery, so than you can have an emergency backup battery 🙂