That said, it's really important to mix it up too. Don't get stuck in the same old habits every time you exercise because you'll find that you plateau. Tony Horton's P90X program gets it right on when he talks about muscle confusion. By mixing it up every now and then you continually challenge your muscles and get more out of your work-outs.
Speaking of that crazy P90X® I've been giving it a bit of a go and as hard as it is, I can't see how it doesn't work. Not sure I'll incorporate it yet, but if I do I'll be sure to report the outcome