The caveat on all of this is
1. If he's healthy and
2. If he's come to play
I don't know the answer to either of those If's, but the first comments coming from the Celtics seem to indicate yes and yes.
Offensively, I think what he brings has already been stated - speed, athleticism, the ability to finish on the break. He will not be the three-point threat Posey was, but he will still require his man to rotate to him or face the risk of getting beat on ball movement or off the dribble. He must be covered (if he's in the right place, which is not, of course, beyond the arc). If the offense is being run properly, he merely has to be a threat if left alone. If his man can't double off him, it increases the probability that whatever combination of the big 3 is on the floor with him (one or two of them will probably be on the floor, even with 2nd unit guys) will be in single coverage. Any one of them in single coverage is a shot I like.
Defensively, his above mentioned physical gifts should have some value if he applies them correctly. Ray (and Paul too) was labelled as a non-defender before last year. Ray managed to play within the team defense sufficiently to accomplish the overall goals of the team (and managed to do a more than creditable job on both Kobe and Rip Hamilton for long periods in the playoffs). Miles has more speed, length, and athleticism than Ray does at this point in Ray's career (although Miles has much less experience and BBIQ). If he can apply those traits consistently, I trust Thibs et al to put him into a scheme where he will succeed.
I'm not sure I'm ready to drink the kool-aid, but I'm starting to feel comfortable that it's not poisoned, at least...