I am finishing up p90x, with about two weeks to go.
That, combined with eating a "primal" diet (read the book The Primal Blueprint for more info), and I've lost almost 20 lbs.
What's nice is that it has been a long, sustained, incremental weight loss combined with significant gains in strength and cardio.
I feel like this is probably a much more sustainable recipe than a 2 week crazy crash diet that drops ridiculous weight but also leaves you depleted, weak, and hungry.
I have done that kind of stuff in the past and it inevitably leads to a very quick relapse back into heaviness.
P.s., I know this will sound cliched, but with the primal diet and p90x workouts, I can assuredly say that I am (a few months shy of my 36th bday) in the best overall shape I've been in since early college almost 2 decades ago.