I've owned my M9P for all of seven days, so I'm not an expert user by any means, but here it goes. The reason why I bought the M9 was that when I had an opportunity to briefly hold one in my hands during a recent trip to NYC, it felt remarkably similar to my M6. I love the M6 because I love the rangefinder shooting experience. I also love its heft, the bright and beautiful OVF and the simplicity of its design.
I find the exact same qualities in the M9. I love that in manual mode you have the exact same arrow display as the M6 and that on auto you have a simple shutter speed display. I love that the menu is only one page long and simple, and that once set up, you rarely have to look at it. Most of all, I love that it is a true rangefinder. If you do not love true rangefinders and rangefinder focusing, IQ is not enough of a reason to pick up an M9.
In that sense, I do not see the X-Pro1 as a Leica M substitute. It is to me remarkably similar to the Contax G2, even in size and shape. I used to own a Contax G system and loved it, but it is a night and day different experience from shooting with the M6. Not better or worse, mind you, but different.
Having said that, it appears that the X-Pro1 is a wonderful shooting machine. It's not a Leica M but it's not supposed to be. To a lot of users, that may be a good thing.
Cheers,
Antonio