It's kind of funny that after playing PO for almost a week and a half, I've literally only seen 2 Mews, and they both sucked (they were both Baton Passers). The fact that it wasn't even nominated for a suspect on PO, which nominated a lot of borderline cases IMO, shows that people aren't really having much trouble with it.
I don't think he's quite the master of Baton Passing now with stuff like Volbeat, Gorebyss, Huntail, and Smeargle having their own uniqe niches as Baton Passers. Volbeat has priority Baton Pass with Tail Glow, which is quite ridiculous. Gorebyss and Huntail can easily pass off a White Herb Shell Break for instant sweeping power, and Smeargle is still the only one with the ability to pass Ingrain and Belly Drum. Speed Boost Blaziken is also notable for basically being Ninjask on steroids. There's also stuff like Stoned Gligar which has Taunt and can pass off Swords Dance and Rock Polish with no problem.
As for the sweeping sets, I understand that the element of surprise can be important, but the thing is, you can only take that so far. There comes a point where you have to think about if the set is reliable, not just surprising. Reliable and surprise is a good combination, of course. I personally just don't think Mew hits hard enough, especially when its only good STAB moves are Psycho Shock, Psychic, and Zen Headbutt, two of which have low base power. The rest of his moves are unSTABed, and there's also the issue that an unboosted 100 base Speed (A Rock Polisher doesn't hit very hard) just isn't enough to actually sweep effectively in this metagame unless you get a ridiculous boost like Manaphy's Rain + Tail Glow. There are too many Choice Scarfers for Mew to sweep very well.
In addition, with a wide movepool and absolute need for super-effective coverage comes 4 moveslot syndrome, which Mew easily has.
Ie. Let's look at a Nasty Plotting set:
Mew @Life Orb
EVs: 4 HP / 252 SpA / 252 Spe
Timid
- Nasty Plot
- Psycho Shock
- Aura Sphere/Ice Beam/Thunderbolt/Flamethrower/Shadow Ball
- Aura Sphere/Ice Beam/Thunderbolt/Flamethrower/Shadow Ball
This is basically the exact definition of 4-moveslot syndrome. He needs to have some kind of STAB so that he can have a reliable attack to things with, and Psycho Shock also helps him against Blissey. If he doesn't carry Aura Sphere, he gets destroyed by Tyranitar and other bulky Darks. No Ice Beam and most Dragons can survive a hit and kill Mew. No Thunderbolt means bulky Waters screw it up. No Flamethrower means Nattorei and the massive amount of Bug/Steels wreck it. No Shadow Ball, and opposing Psychics as well as bulky Ghosts screw it up.
My point is, while Mew can be surprising with his boosting sets, just remember that he does need to take up a slot for the boosting move, and he's going to get screwed by neutral targets if he doesn't carry some sort of STAB. This means that he only has 2 other slots for coverage, so the element of surprise doesn't last very long, and even then, many things can still take a hit from Mew, not to mention that Choice Scarfers will give it hell.
This isn't to say that Mew is a bad pokemon, I just think that he's more on par with the other base 100 pixies this gen. I'd personally rather use Jirachi any day for sweeping though. Or Celebi to Baton Pass Swords Dance, Nasty Plot (remember that he does get it via event), or Calm Mind.