This is far better for a nickname than Bob, Rob has more strength, also, in a lot of TV shows and movies the simpleton character is often named Bob, so now if I think of a Bob I can only think of simpletons.
Of all the nicknames one can get from Robert, this is probably the one I find the most agreeable-sounding. (Though, I think that the name Robert is best used on its own, with no nicknames or anything.)