I have the regular P90...it's pretty good... definitely good if you''re not in the best of shape like I was due to a knee injury
For P90X I would suggest that anyone with knee problems be very careful. There is alot of jumping and quick movements involved in the program,which they even warn may be bad for people with bad knees and joints
Also there is a P90X Plus program out there as well. It's basically an add on to the P90X program. The works out are 40 minutes, there are less breaks andmuch tougher excercises but man does it make you feel good afterwards.
***Just remember, don't be discouraged if you can't even come close to keeping up with them. After a week or two you'll be doing more pushups andpullups than you ever imagined you'd be able to do***