Manuel,
To address some of your concerns, it might be interesting to know why you'd want to switch from the M3. If you'd like a bit more exposure automation, consider the M7, although you can't use a DR lens with it....
But, with the G2, you do get AF; a significantly higher shutter speed, so that you can shoot with maximum bokeh even in bright lighting situations, faster film loading, and film advance. The focusing 'issues' are, indeed, significant, even though the advice from advocates will tend toward: "make sure the rectangle is on what you want to focus on...." But, still, as someone who felt i understood how to operate the camera, i still had more than a few frames that weren't focused exactly where i intended. I'm not talking about having a person out of focus, and the wall 10 meters behind In focus.... I like to shoot wide-open/with shallow depth of field, especially with people. With the G2, i would sometimes get slight shifts in focus because i was re-composing to non-centered composition after locking focus. Of course, with a Leica, the same can happen if you don't adjust the axis of the camera to compensate for the focus shift, but in practice this never happened with the M7. The truth is, you're never as sure with the G2 of what will be focused. In practice, this is seldom a problem. I don't consider the G2 to have focusing "problems." Perhaps it can be considered a "quirk," but as it is said, focus problems are usually the operator's fault. That said, i would still, always, prefer a system that shows me in the viewfinder what will be in focus. And, with that admission, i would still consider buying (again) a G2, because i understand the 'limitations' of that system.
About focus speed - yes the lens racks to infinity after every exposure. But, still, it focuses faster than i ever could with a Leica. I don't consider it ideal for fast-moving subject matter, but then that really has never been the domain of rangefinder(-ish) cameras. Use an SLR for that. But, the G2 is probably better for shooting children, for ex&le. I was never very good at shooting children with the Leica, unless i was satisfied with keeping them in the center of the frame, and expanding the depth of field....
I've never heard horror stories or any bad comments about the G2 metering. I consider it accurate. And, quality control? No issues with any Contax or Kyocera product. Customer service, though, may be another discussion. In the US, it's not good. I've read recently that in Japan, it's excellent.
Well, that's my "synopsis." Hope it helps, but don't take any of it as 'truth.' Just my impressions, having worked with both systems.
Good luck
Derek
www.derekstanton.com