Correctly written LWPs don't kill your battery... often though, it is trial and error. I recently picked up the S3 LWP (the dandelion one) free in the Play store -- looks great on our screen and doesn't hurt the battery.
example on HTC Desire (I don't run it with the water effect)
[YT]http://www.youtube.com/watch?v=chKMrjm63u0[/YT]