Is P90x a good workout if you don't follow the 90 days?

In short, no. The workout is based around, not just the exercises to work the muscle groups, but in maintaining a higher heart rate like in circuit training. Most of the exercises are solid (curls, pull ups, tricep extensions, deltoid raises). However, any fitness mag can give you a better workout plan for straight lifting. You should start out doing chest and triceps on monday, back and biceps the next day legs and shoulders the next, then take the fourth day off. Once you plateau you can go to muscle confusion and go chest and biceps, shoulders and back, legs and triceps. Plateau again and go back to the first setup but switch out the exercises, like going from doing barbell shoulder presses and bench presses and switch to dumbbells. The dumbbells will also work the small tendons and balancing muscles that don't get worked as much using a barbell.