I've had my MM for more than 7 months now and it hasn't failed me ever. It syncs right away when I boot up, the right click works fine with no issues at all (I don't know why so many people have problems!) the scroll ball was a little weird at first, but now I love it and regular scroll wheels feel very clunky and old to me. Also, the side-squeeze function for Expose is something you can't live without once you start using it.
Some people complain that the scroll ball doesn't work well. I've had times where probably a spec of something gets in there and it won't scroll up or down, but I just run it over a damp cloth a little, as Apple says to do, and it works like new again. I think the complainer just don't try to clean it. Some have even mentioned taking it apart to clean if it gets really bad, which sounds fairly easy to do.
The best part is no wire or even USB adapter and being a very nice low profile design. I can easily toss it in my bag with my MBP without it taking too much room and use it on any surface or in the classroom for easy navigation while others have to deal with the trackpads.
I was hesitant at first too from all the complaints, but I haven't had one significant issue with mine.