yumy I have Insanity. To be 100% honest, it is not something I would recommend to beginners, but it's brilliant to work up to.
Even when I could run an hour straight on the treadmill with lots of sprints, I would still end up collapsing in a sweaty heap before the end of an insanity workout!!
Jesus, the people they have in the background doing the workout clearly struggle and have to take breaks, and they are spinning instructors, marathon runners, etc.
My personal view is that it is too hard, and too daunting for a beginner. If you struggle at all with level 2 of the 30 day shred, then don't go near Insanity with a barge pole.
HOWEVER, that said, there are lots of people who do start with insanity, but just have to take it really slowly to begin with. Slowly, lots of breaks, push yourself, but not so hard that you start feeling ill.
This is all about the insanity workouts though, which are 40 minutes long, progressing to 60 minutes. I haven't tried the T25. My understanding is that they try and cram in a workout into a shorter time frame, so will probably do lots of very high intensity interval stuff, or moves which burn a shit ton of calories, like vertical jumps and tuck jumps. And everyone's favourite - burpees.