I have some aerobics class experience so most of the moves were familiar to me, but even so, some of the older (80's & early 90's) FIRM DVDs go too fast for my taste.
I like the ones from the Body Sculpting System 2 (the first ones with the "sculpting stick"), although sometimes I still have to pause it when they're moving equipment around, I don't think I'll ever be able to set up as fast as them! lol. They tend to start out simple (sometimes with explanations/definitions) and add on the more complicated moves, instead of just plowing right into it.
I haven't done anything from the newest series or two, so I have no advice on those.