imo Robben is an average player with one extraordinary move (this run on the right side then go in and shoot with the left foot). In the moment the defensive lines learned he mostly does this, they know how to defend him. In the meanwhile he also lost a bit of speed, so he can't overrun the defenders anymore that easy.
He is one dimensional and has no right foot, but that one dimension is still pretty damn good, and defenders still struggle to handle him, despite knowing what he is going to do.
He has proven himself in 4 different leagues in Holland, England, Spain and Germany which is very impressive.
He was a major contributor to Chelsea's success in 2004/2005 when they won the EPL with a record points total (most of their creativity that season came from him), and in 2008/2009 when Real Madrid were very bad for their standards, he played very well and a lot of RM fans were sorry to see him leave. For Bayern his goal-scoring record has been very impressive although he is selfish and doesn't create as many goals any more.
In the second leg of the RM-Bayern CL semi-final at the Bernabeu last year, he was absolutely brilliant.
That being said I do think that Bale is the more talented player of the two. Of course it is ridiculous to compare Bale to Messi or Ronaldo like a few people in the media have, but a Bale-Robben comparison is more appropriate I think.