I love live wallpapers, so long as they are simple and not distracting. My favorite of the ones I've tried is Pixel Rain. Light Grid and Exodus are good too. I have not noticed any detrimental effects on my battery life while running a live wallpaper. Some of the more intensive ones, or less well designed, may hit the speed a bit, but the three I mentioned are all great.
Another one here for using LWPs only... Good ones don't put a hurt on your battery and don't affect phone usage; however, bad LWPs will kill a battery so fast you'll swear your battery meter is a countdown timer for NASA.
Have had lots of luck with LWPs from Kittehface -- they look and perform fantastically!