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Mega evolutions are quite useful when used at the right time. Its mostly all about powering up abilities in a clutch situation. (:
I happen to use mega evolution whenever I can to end battles quickly.

As far as aesthetics go, Mega Evolution is quite an addition. Like everyone knows, one of the most thrilling things about Pokemon is and always has been evolution. Nothing like your Kanto starter evolving from its cute form to its tougher teen-looking form, and then finally that ferocious 3rd Stage and utilizing its new found powers! Mega Evolution gives you the thrill of that one last power boost, making your Pokemon look tougher than ever!

At a Competitive point of view, Mega Evolution isn't all it's cracked up to be. For example, you think you gain the element of surprise since your opponent doesn't know which Pokemon holds the Mega Stone. But let's say you have a T-Tar and Scizor, your opponent is going to assume Scizor holds the Stone because his Mega is generally better. Let's say you have Scizor, T-Tar, & Mawile, your opponent is going to KNOW Mawile is holding the stone because regular mawile blows.

The only Pokemon you'll see often in Standard Battles w/o a Mega Stone that can Mega Evolve are Tyranitar, Scizor, Garchomp, Gyarados, and maybe the rare Alakazam or Scarf'd Heracross. But of those, the only common one used for Mega Evolution is Gyarados, because he gains considerable bulk and just runs a D Dance set anyway. I have sweeped so many unprepared teams by leading with Mega Gyarados, but only if my opponent has nothing to counter it. T-Tar & Scizor basically either lose Leftovers or extra power that could be added through Life Orb. LO T-Tar/Scizor hit harder than their Mega forms, because they have no item to utilize and the LO boost is just a better boost. But Mega Scizor has Roost so meh. Garchomp's mega is slower, and most people just prefer regular Garchomp. Mega Heracross is in the same boat as Garchomp. Mega Alakazam really is just a faster, less powerful Alakazam. And tbh, 120 Base Speed is enough, the power boost from Life Orb is far more important. Or you could run the Sash-Revenge set. Magic Guard makes both of these possible. Trace is gimmicky too.. Which MegaZam has to use.

Any Absol, Pinsir, Mawile, Ampharos, Manectric, Gardevoir, Medicham, Venusaur, Blastoise, & Charizard will almost always hold the Mega Stone. Otherwise they're completely terrible. If you predict any of these incorrectly, you still have the advantage because their regular forms are so terrible. Idk if people use Mega Aggron, Houndoom, Aerodactyl, Abomasnow, or Banette, I've never seen these used.

Re-cap: Some Mega's are just a wasted use of your 1 Mega, because the regular form is just as good/better. Some Mega's are still bad, and meh. And then there's the ones who are good, but totally obvious. Those are the 3 categories.

One last note, I love the fact that Mega Evolution has actually brought many terrible, but loved Pokemon into the competitive scene after generations of NU. Good job, Nintendo. pirate

EDIT: Inb4 Brokenly Defensive Mega Sableye w/ Prankster spam.

Ah, of course! I had actually thought of that but was focused typing out some other thoughts and I forgot to put that in as a last note. But yes! That is the one element of surprise factor and it has costed me as well. I lost when I predicted the Dragon variant, but when I expected the Drought one I still ended up winning that one. My opponent just got too comfortable and tried to set up too much, otherwise he would have gotten a decent sweep and inevitably won.

Best advice for it is to see if there are any Sunny Day teammates. But be prepared for both variants either way.

On me it depends on the case and with my team comp, since i run very tanky pokemon ie Togekiss, Mandibuzz, Venusuar, Chansey, Tentacruel, Gourgheist. I use a very offensive mega that can benefit from such tanks like Medicham, and Mawile . But we all know when ORAS comes mega Sableite s going to get banned