RE: I'm an old Firm Workout Junkie, If I convert where ...
I also started with the Firm and "graduated" to Cathe. I had been doing the Body Sculpt & Cardio Sculpt workouts for a few months and then added running back into my exercise program as my primary cardio. I thought another strength training DVD or two would be good for variety. I kept reading & hearing great things about Cathe, so I ordered the GS series and Muscle Max. Those are still my favorites.
Since I run and kickbox, I can't really give a recommendation on the step workouts. I'm not that coordinated and tend to avoid them. (Tried Imax2 and it was just too much for me.)
Whether you're doing cardio or weights, I think you will definitely find her workouts more challenging than the Firm, but just do what you can in the beginning. You will DEFINITELY get results!
I don't currently belong to a gym. Since I've invested in the equipment and DVDs, it doesn't seem necessary. It's extremely convenient to work out from home, and I change it up a lot so I'm never bored. If, eventually, I get sick of it or find myself not being challenged, I may change my mind, but for now, I don't see a real need (even though I live near Cathe's gym and I do find myself very tempted to join!)
I don't have the Transfirmer (I used the ol' Fanny Lifter), but I think it should be fine for Cathe's workouts.
ETA: You should be able to find all of her workouts on this site.