JCore - Thoughts? Reviews?

I've been looking into at-home workout routines that I could do on a tight schedule that wouldn't (1) kill me, (2) require a lot of space, and (3) need a bunch of equipment. Oh! and (4) cost a fortune.

I want a tough workout (I want to feel it the next day or so), but at the same time, I don't want it to leave me so sore that I can't workout or am hobbling around (my knees aren't so great). My schedule changes depending on the project that I'm on, so sometimes I have to either work late or get up early - which means I miss a workout.

I've run across a few and thought I would see what you guys thought? I was leaning towards JCore because (from what I've read) the workouts are 20 min / 4 days per week - which really fits my schedule. If I am reading things right, the routine changes so I won't get bored and I will be hitting muscle groups differently. And, there is no equipment.

Others that I've seen is Insanity, P90X (equipment required) and Rush Fit (don't really know much about that one).

I was a little concerned that either Insanity or P90X would be too much for me since I am out of shape, but wasn't sure.

Can anyone give me some advice on these?

And, should I do cardio (i.e., elliptical) at the same time as the workout?