It's time! Last night, Niantic released the Mega Evolution update globally, giving all of us the opportunity to take full advantage of the revamped system.
Our database is already being updated with all of the new updates, but the announcements don't stop here. It seems that we've not only got Mega Kangaskhan, but also the grand release of Legendary Mega Evolutions with Mega Latios and Mega Latias!
While we haven't seen a "global" announcement for the Mega Legendary Pokemon yet, an official video posted to the German Pokemon Go Youtube page, strangely in English with German subtitles.
So now comes the big question: how do these two stand up to their competition? Well, because Mega Evolutions still can't join the Go Battle League yet, they're only really worth considering for raids right now. To that end, Latias is virtually worthless due to its lower Attack stat. Latios, on the other hand...
Psychic Performance
Dragon Performance
So overall, Mega Latios is set to join the game as the best DPS Mega Evolution for both Psychic and Dragon Types. However, it's also destined to fall into obscurity in the future as even stronger Mega Evolutions take over these rolls. That said, it's definitely worth raiding for a good Latios to Mega Evolve for the moment, and we can even see more utility coming in the future with its Signature Move, Luster Purge, being recently datamined. Note that we won't rate it yet, as it may undergo changes pre-release.
On that note, Latias' signature move, Mist Ball, has also been datamined. However, this isn't going to help it very much on the PvE side, at least.