I don't think the M is the best mirrorless by any stretch. Certainly you can do better with a Fuji or Sony. But for the price, since the recent drop, it's unbeatable. With the kit lenses the AF is fine - don't believe the fud. With EF lemses via the adapter, it really depends on the lens. Some are much faster than others. Pre focusing helps.
Now, the question is why would you want to put this little M on a big lens - what's the point?
1. Touch screen. Very cool. I can definitely see this tech coming to all cameras. Canon has their UI down, and puts all others to shame. Anyone who poopoos the touch screen hasn't spent enough time with it.
2. Shutter sound. Virtually nonexistent. This is why I bought the M, so I could use my EF-Llenses and shoot with no mirror slap. No more dirty looks while shooting performances.
3. Weight. Well, this is more theoretical than anything. The M is really light, but horribly off balance with a big lens attached. The EF-EFM adapter is very nice, but they should have incorporated a grip into it. I'm waiting for someone with a 3d printer to put one up on kickstarter.
So, should you buy? Always a touch question. If you can get it for $400, definitely. Otherwise I'd hold off until the next gen.