At first I stayed away from live wallpapers to save battery, but now I've been using the aquarium for about 10 days, and it's only 3% of the usage. Seems worth it to me. Thats way down in the noise-level compared to the power consumption due to my office being in a bad reception spot.
Using the Super Mario one myself - he just got stuck on my screen too - but the easy fix is to just go in to the wallpaper configure and tell it to skip the level - it starts a new level and no more stuck mario!
I haven't noticed any adverse battery effects from running it either
EDIT: The seeds of life one just looks like a bunch of sperm swimming around on your screen?
I don't think the live wallpaper can drain the battery, or slow the phone, except when it's actually being displayed. If you're in another app, it's going to be inactive in the background like any other app not being used.
Even using the Mario wallpaper, which must be the most cpu intensive one there is, I don't see a slowdown when scrolling through my apps under the settings menu, or opening or switching between apps.
Edit: Using the System Panel app, and looking at real-time cpu usage, the Mario wallpaper appears to use zero cpu when it's not actually being displayed. If it's not using cpu, it can't slow any other activity.