I shoot in A about 85% of the time, M about 10%, and S for the balance. I don't have anything against shooting in P (i.e., I'm not one of those people who looks down my nose at people who use it), it just gives me headaches 'cause the "choices" the camera makes sometimes aren't the ones I would've wanted it to make under those circumstances. And if you're going to change your aperture or shutter speed in P (which I've always found to be a pain), why not just shoot in a priority mode anyway?
The great thing about M (once you've got the hang of what shutter speed, aperture, and ISO each do for your exposure) is that you can push the camera past what the built-in meter thinks your photo "should" look like, and you can decide what compromises you're willing to make in terms of under- or over-exposure. You can still do that in S or A* as long as you know how the different parts of the equation relate to one another; M just streamlines the process, and gives you a quicker/finer degree of control.
It's especially useful if you're shooting at night or in low/dodgy light, since the camera's always going to want to expose for middle grey, which has the end result of making everything look like it was shot around 2:30 in the afternoon. ;) I like my night photos to look like they were taken at night, which is where M mode really comes in handy.
*Or using exposure compensation, but then you're stuck if you want to go past the EV your camera allows while using it