I use the 608 for walking primarily. I have avoided using it for The FIRM, primarily because it doesn't have a pronounced heel, which I think is best for The FIRM. (Leg presses are supposed to be done with a focus on putting your weight on your heel -- try that in the 608 and you'll topple backwards! <grin>) The 608 is also a bit heavier than the other two New Balance shoes I have.
The 810, on the other hand seems to be pretty good for aerobics. It's light, and has a heel. The only problem I have with it is that it is harder to make the lower-section of laces tighter, which is best during The FIRM. As a result, I find myself putting weight on my toes a bit more, because I don't trust the feel I get sometimes (like when doing lunges) by just relying on my heel.
The 825 is a much lighter shoe. I resisted using it for The FIRM because it has a really wicked tread, so it would eat up my carpet. I've gotten over that and it's a really good choice IMHO. I can tighten it well, all the way down, so it really feels part of my foot ankle to toenail. The heel isn't bad.