How much does it cost? £106 for the DVDs, plus extra for mats, dumbbells and pull-up bars.
What does it promise? "A revolutionary system of 12 sweat-inducing, muscle-pumping exercises designed to transform your body from regular to ripped in just 90 days." It works on the principle of muscle confusion, where your body isn't given time to adapt and so continues to improve.
What's it actually like? The man in charge of P90X is the impossibly cheery Tony Horton, a sort of cross between Jim Carrey and a hot-water bottle full of metal lug nuts. He guides you through an initially impossible-looking set of exercises, including towel-grip pull-ups, one-armed push-ups and a ridiculous number of bicep curls. However, all the exercises can be modified.
Best and worst bit The highlight is the Ab Ripper X programme, which involves lying on your back and climbing your own leg like a cat chasing a squirrel up a tree. The problem with the concept of muscle confusion, I quickly discovered, is that you're essentially signing up to three months of waking up to a new and subtly different type of pain. Walking downstairs a day after a leg workout, in particular, is a special kind of torture that you should discover for yourself.
Is it worth it? A stranger at my gym saw me doing towel-grip pull‑ups and shouted "P90X!" at me. He then told me that I'd be "beasting it" by the end. So, if you like that idea, then yes, it probably is worth it.