Coming out of lurkdome (like way too long's worth of lurkdome--is that a word?) to say that I too have been revisiting my classic FIRM's this past month and have been loving them all over again. Specifically, I've been mixing Vol. 1, Vol. 4, Tough Tape, and Tough Tape 2 with Focus T25 for cardio. I'm sure I would have great results with this schedule if only I got my exercise consistency back on track.
As a side note, I wore my heart rate monitor during both T25 and Vol. 1/Vol. 4 to see how much of a difference there was between the two programs and my heart rate ended up being so much higher for a longer duration with the FIRM compared to T25. I guess I forgot how efficient those FIRM's were!