Webber is a very high quality driver, when he was recruited to RBR initially it was to lead the team over David Coulthard.
Webber’s reputation over the years have been destroyed by poor team change moves and his rather inconsistent results.
Mark has always been a superb qualifier and as people have stated before has always had the upper hand over his team mates until one Sebastian Vettel. Last year and in 2009 he put some devastating results in (Nurburgring 2009, he got a drive through penalty and still emerged in the lead).
It is difficult to say if his pace is dropping off as he ages or if Vettel is simply that good, but Qualifying no longer seems to be his strong point (and it used to be something he done very well).
This year his problems are compounded by his starts, he usually loses atleast a couple of positions off the line and this means he spends most of the race fighting back and destroying his tires. Also towards the start of the season he was seemingly unable to make his tires last as long as vettel for whatever reason.
I will concede that Vettel is a faster driver this year, but Mark still has the superior race craft, some of the moves he has pulled off this year are just masterful.
He has had some bad luck in his career (remember Melbourne 2010? He was running second but had to wait for Vettel to switch to intermediates, and by the time that happened he was burried in the pack, that is a race he could have easily won).
This year being hit by hamilton at Canada, having a 40 second long pit stop at Monaco and not pitting under the safety car at Spa are just a few reasons why he seems to be forever behind Seb. There are plenty of races he could have won but frustratingly for him it just never seems to come all together.
Marks season has gone like this, qualify 2nd on the dirty side, get an average start, end up 5th, fight your way back to a podium, watch as Seb drives away from pole.
All in all he is still a very good driver, definitely above someone like Massa. 12 months ago I would have said faster then Jenson (but less consistent), but Jenson has shown supreme form lately so its hard to say.