Right now there is no evidence for strings or membranes or anything else. I think this is hard to argue with. But that isn't really very surprising, there just isn't any evidence period. There's a decent chance we will never be able to probe the energy scales of quantum gravity.
Of course, string theory or M theory seem consistent and reduce to general relativity at long distances and low energies, so they aren't ruled out. But we haven't yet seen membranes, or strings, or extra dimensions, or even supersymmetry.
Note that this is logically unrelated to the consistency of M-theory. M-theory has been useful theoretically, and it's possible that it describes physics in our world through a kind of back door called holographic duality, but even this remains to be seen.
So to answer your original question, I think the theory is credible. It's certainly beautiful to many people, including me. At the very least it offers some interesting and potentially useful insights of a theoretical nature. Still, I'm often doubtful that we'll ever be able to really test whether there are M2 branes or whatever.
