I've always been a traditional weights, heavy lifter, and at the beginning of this year I did a rotation of Pink Firm circuit workouts, and have been doing circuits of different instructors since then, all year, and haven't done anything really "heavy" all year.
Anyway, much to my suprise I found I really like it! I get more of a fully-worked-out feeling because of the cardio. And as far as increased fitness, there are plenty of advanced workouts that have tough cardio, like IDK, Cathe's Bootcamp, Get Ripped 1000, stuff like that. I can never even finish all the circuits in Bootcamp! You were talking about wanting a challenge, and this type of workout can *definitely* be a challenge! And that's where the increased endurance comes from.
Results: I don't think I've lost stregnth. I just started CLX yesterday for the 1st time so I'm just getting a feel for what I can lift at a slower pace right now, but I'll be picking a heavier weight next time cuz I definitely underestimated my stregnth.
And there was a vacation this summer, I gained 6lb! and lost it all when I got back home. I think it does have a good "metabolic effect" for me b/c it seemed easy to lose the weight. Other than that, I've been maintaining weight really good all year. And I still see my quads, which is what I use to judge my over "muscley-ness" - If I don't have visible quads I think, "Hey I haven't been working out enough, they've disappeared!"