First: no distro is bug free, that's a fact and unavoidable in practice. A nice saying goes: "If you can't find a bug in your software, try again!"
As somebody who has been using a lot of distros (and still does) I think Mageia is not old enough to compare it with others of the BIG ONES for this question.
Mageia 1 had the advantage that it mostly consisted of packages ported from Mandriva - of course the people who ported the system only selected packages that had already been checked for bugs before and did not port packages which had an unresolved bug. This resulted in a distro with as few bugs as possible - the hard work by the makers of Mageia did the rest. Let Mageia grow as "old" as other distros and we will see how "bug free" it will be in 3 or 4 years. Then it makes sense to compare in a fair way.
Yes, Mageia is stable and it has less bugs than many other distributions. But how "bug free" Mageia will stay in future depends not only on the good job the developers and packagers of Mageia but also on the quantity of manpower we will have for QA. Join the QA team, participate in testing and give usable feeback to help resolving bugs - this way we will continuously improve Mageia's standing in the "bug free" ranking
