If anyone doubts Marquez's ability then I recommend a trip to Specsavers (for those not of these shores and unfamiliar with Specsavers that's a chain of opticians).
Alternatively, one can always watch a host of past senior class motorcycle Grands Prix and the newer MotoGP series. Those with sharp eyes or a decent optical prescription will note that one of Marquez's key talents is to observe, learn and improve on the techniques of great racers of the past and his peers. Commentators often remark on how he reminds them of someone else like Doohan, Spencer or Rossi for example.
The previous youngest (and rookie) bike GP champion "Fast" Freddie Spencer took the "knee on the ground" technique to a new level by sliding further off the side of the bike than those who went before. He was soon copied by his peers. Marquez has done the same with the "elbow and knee on the ground" technique. Lo and behold, already Marquez has influenced those around him. Until he came along no-one sustained the elbow on the ground for more than a metre. Indeed, even in Moto2 Marc's peers had not yet emulated him. But once they saw how quick he was at the start of this season the top half of the grid have been trying it. The results are mixed with a lot of the guys falling off because they had not got the timing right or got enough of their butts over the side of the bike.
Another skill he has is the speed at which he learns a track and the method by which he goes about that learning. That also must apply to how quickly he mastered the Honda. The factory Honda and Yamaha bikes may be the class of the field but they are brutal machines that would dump all but the best of racers on their arses at the first twist of the throttle. The same goes for getting one of those things around a track at any sort of decent lap time. It isn't simply a case of having one of the best two machines on the grid, Marc has had to control a beast that will - and indeed occasionally does - spit him off at a microns worth of miscalculation.
There is an argument that we don't know how good he is because his main competition threw themselves down the road and hurt themselves more than he did. Well, don't forget the adage that to finish first, first you have to finish. Only the one questionable off - i.e. Dani's following the merest touch of a poorly located traction control sensor by Marc's elbow, apparently - was down to anything other than the blokes themselves chucking it down the road. Apart from the iffy sensor location (questionable), none of his direct competitors DNF's can be put down to mechanical failure. So, in my book that one doesn't fly.
So, as to the question regarding how good he is I would ask "How good is this kid going to get?"