I've got an M4. It's an Aimpoint. Not much else you can say. Tough as a brick and very quick in use.
Presently, mine has the QRDII mount. It seems OK so far, but I've heard reports of this one needing some serious locktite for serious use. I've been planning to pick up a Larue mount to replace it, but simply haven't been able to spring the funds to make the switch (too many other projects going on).
Battery life is stupid long on the thing. Supposedly, it's 80k hours on--I think--setting 11, which is about 2/3 brightness. I just put the batteries in the thing and turned it on and have ignored it ever since. Probably a couple of months now and it just drives on.
The general consensus is that it is an improvement on the M3, but not a huge one. It does have a built-in mount, but it's not as good as the new Larue one. Larue is selling M4 packages with their mount for the same price as the Aimpoint QRDII package--helluva deal. There are no rings; it's bolted directly to the mount. AA batteries. Better sealing. An improved switch (forget exactly how, but it's supposed to be better). Improved adjustments. Just depends how much that extra $120 it costs is really worth to you.