I have no experience with digital M's. My Leica experience was simply with an M6 and a 35/2 "bolted" to the front.
That being said, I can say that my X100 would not only be a backup to the M6, it replaced the M6 by it's flexibility and high performance. I'm not truly looking for an argument, it's just my opinion.
Based upon tech specs alone, if you're only shooting 35mm FoV, an X100 will retire your M8 and be equivalent of your M9 (functionally speaking). If you're shooting RAW only, you'll only see differences to M9 and X100 files by how they're processed. The jpeg engine is astounding on the X100 though, if you ever shoot like that.
The X100 offers this flexibility: the ability to shoot macro, and a great fill-flash...neither digital M offers that.
All this being said, the X100 won't duplicate your "Leica rangefinder experience." The X100 is NOT a Leica. I find it's close-enough in actual operation, though, that I don't pine for my M6 personally. My favorite part of shooting an M was holding the damn thing in my hands, I'm embarrassed to say.
So...the point of my diatribe? The X100 will make an terrific backup, or a terrific primary shooter. If you shoot 35/2, it'll rival anything you can make with an M body, film or digital. Yeah, big words, I know.
Here's a thread I started with a few shots I made this past week in Hawaii with the X100...I'm perfectly satisfied by it's abilities as a modern camera, with a 70's-vintage feel...
http://www.rangefinderforum.com/forums/showthread.php?t=112695