Federer doesn't get enough respect for his backhand I feel. He can guide the ball with it very well down the line or crosscourt, he is a tremendous shotmaker on the backhand side in awkward positions and comes up with all varieties of brilliance from it after the ball has barely bounced or on the full stretch with a flick of the wrist and such. He can sometimes shank it but whenever he plays these guys who people speculate have a better backhand such as Gasquet or Wawrinka or Murray or whomever, he more often than not gets the better of them in the backhand to backhand exchanges. I think the fact Federer sometimes shanks the ball a bit actually distorts peoples view of his forehand.
Perhaps somebody like Gasquet or Haas do have a better backhand than Federer, but I do not see it. They lack the ability to neutralize a rally they are behind in with backhand defense, which Federer does uncannily. Gasquet is explosive and potent with his stroke, but partly due to the mechanics of his stroke he ends up camping out too far behind the baseline in order to make the best use of his stroke, it's a bigger backhand than Federer's but not as 'vast'. If pressed to choose a backhand to possess out of these 3 guys, I would probably choose Federer's. Other great contemporary backhands would include the backhands of Wawrinka, Kohlschreiber, Murray, Nadal, Nalbandian.
From watching the AO I can say that his basic rally shot had excellent depth and he consistently made tough and excellent shots with the backhand all tournament long. Against Murray his backhand was actually a wand, he often murdered Murray with precision hitting with the backhand, exploiting the spaces of the court effortlessly and brilliantly.
Federer can hit over the ball very well, hit it flat or knife the slice, he has tremendous backhand defense with the slice lob, can hit the top lob and also has an excellent drop shot. He has a super strong wrist, clearly one of the strongest in the history of the sport, which allow him to guide the ball with amazing precision very often when stretched out on the backhand side, or to control half volleys even with the ball rattling at him at 100 mph. He also has that consistent, powerful and excellent backhand overhead/smash/whatever that nobody else on the tour seems to be able to produce with the same effectiveness or venom as Federer.
Federer's backhand isn't close to being one of the most brutal shots in history like say Kuerten's or Laver's were but it is the backhand with the most versatility I have seen since watching tennis more seriously from the mid 90's onwards -- so that is 15 years. Basically, when Federer is flowing the 'vastness' of his options trump the vast majority of other players and of course when he is on song his possibly 'GOAT' forehand allows the space and freedom for his backhand to be even better.
In Federer's prime also he had such confidence in his backhand pass that he constantly made the play to bring other players in on short cross-court slices in order for him to have the opportunity to make a pass that he was sure he would make.
It's a tremendous shot and I believe markedly better than Sampras' backhand, which was also good.