I must say, I'm a sucker for parts/compilation workouts. Fitness Favorites has done this well over the years with superb chaptering.
In reference to the FIRM, I don't even want to admit or try to figure out how many times I've traded them off, only to regret it terribly. I've never had a huge workout collection to begin with and limited funds, so trading is what I would do in order to try something different. As I age, wisdom has set in a bit, so keeping them is probably a wise choice.
I have the entire WHFN Series and never plan to part with them. I believe Anna did some of her best work with these.