I've tested it extensively now. It's great. That's been covered. It's really true. It's probably the best Pokemon out there when you just need a very specific niche, that is, of a bulky Baton Passer/Set up sweeper. Does it do any particular thing in a way that nothing else in the tier does uniquely? Eh, not really. Well, I had a post about this, but it's since been deleted because I derped on a format.
First things first:
@Machi: CM Meloetta WILL beat Lefties Mew 1-on-1. Psyshock is a 3HKO at +2, and that's just a bit more than a +2 Mew will do to a +1 SDef Meloetta with Dark Pulse, so there's that point gone. Inversely Meloetta can easily 2HKO unboosted, and has a hefty chance to KO at +1 after any sort of prior damage, be it Spikes or SR, or even some other Pokemon smacking it with a neutral move and getting out straight after.
While Meloetta may not be able to swap directly into Mew, Mew definitely can't in return - and if they swap in at the same time, Meloetta is guaranteed to beat the stuffing out of Mew by using Calm Mind twice and OHKOing cleanly with Shadow Ball while Mew can do a 3HKO at best after factoring in Meloetta's Leftovers. Hell, not even a +6 Psyshock has a chance to OHKO full health Meloetta without Life Orb - and remember, LO means that Mew will be dying at +1. If NOTHING ELSE then they are at least checks to each other. Let's drop this once we've settled that they can in fact beat each other given the right circumstances, and that said circumstances are likely to happen both ways.
I'll acknowledge that the current metagame often has teams packing 2 unintentional checks to Heracross... but is it because it's a centralizing factor of just because, in the end, it's not all that hard to check it? I mean, any scarfer above base 85 will do the job nicely, as will most bulky waters excluding Blastoise and Suicune... it doesn't have to be an defensive check, remember.
I will agree that is its incredibly easy to tailor-make, but that's not necessarily a bad thing. It allows for more interesting tactics than the metagame would otherwise allow, as Mew can be either a ferocious attacking force or the ultimate team supporter.