Although the hype for it when it was first announced was strong, it inevitably died down and left Mega Slowbro in an awkward position where many insisted it was hard to justify it's use over base Slowbro, while countless others viewed it as an unbreakable monster that was initially ban worthy iirc.
Mega Slowbro has started to gain more traction in the metagame and it'd be interesting to see other opinion on it. Mega Slowbro is a behemoth with 95 / 180 / 80 defenses backed by an impressive movepool. However the main reason I've decided to touch on Mega Slowbro, is that it's in my eyes one of the better defensive Mega Evolutions that fits very well on offense and stall, while it's full potential on balanced teams are generally weaker than that of the former two. It has such a great place on these teams as it base form is fully functional as a defensive pivot into the likes of many powerful threats rising to power such as Offensive Garchomp and Band Terrakion who's been super popular lately. It's Calm Mind set, which is arguably it's best set, is very reliable wincondition and nearly unstoppable after a boost. Any counterplay involves Special Attackers that hit it super effectively or massive damage such as Gengar and Latios or reliance upon wearing it down with Toxic.
Although I'm not a big stall user, Mega Slowbro stall has certainly been the more popular and solid Stall teams rather than generic Mega Sableye stall. The common issue that arises with Mega Sableye stall is that there is usually the lack of a solid way of securing a victory, making it easier to take advantage of than Mega Slowbro stall. The main problems I see with stall, is that wallbreakers can wear them down to such a low standing that setup sweepers can potentially get through with little to no trouble. Mega Slowbro gives stall a slight nudge in the correct direction in having a reliable wincondition that can hold it's own against a bevy of wallbreakers.
In regards of offensive archetypes, Mega Slowbro again offers itself as an incredibly reliable wincon that takes offense another step to proving that bulky offense is the best offense in the metagame. Common offensive breakers such as Mega Lopunny and Weavile sort of just kneel over to Mega Slowbro. Expanding upon the latter, Mega Slowbro effortlessly walls even Physical Attackers that hit it super effectively unless they have some method of boosting or access to Taunt, if not they get hopelessly walled used as setup fodder and it becomes nigh unbreakable. In other words it usually just negates Physical means of taking it on, not to mention that even if they have a method of boosting Scald burns do exist and could simply make that answer obsolete.
Other observations I've noticed is that Sand will remain a top tier playstyle perhaps until the release of the upcoming Pokemon Sun and Moon. It's just becoming such a common aspect of OU, despite it's ubiquity of checks such as the increasingly common Rotom-W and Landorus-T.
Physically Defensive Tangrowth is all the rage now as it's ability to blanket check nearly any Physical Attacker in the tier is a huge plus in it's direction, despite being KO'ed by nearly all Special Attackers in the tier.
I haven't seen too many Mega Charizard Y around and I'm curious why. I'm assuming that the sole role of a wallbreaker while under Sun isn't valuable anymore especially with Sand becoming the dominant weather and there are actually other wallbreakers in the tier who just do it better.
Mega Diancie seems to always be the go-to Fairy for offensive builds as it offers a large degree of power and speed while also having solid bulk and a defensive typing to allow it to blanket check a few threats to offensive teams such as Talonflame and Tornadus-T.
I spoke on Band Terrakion earlier, and it's incredibly viable in today's metagame as a terrifying wallbreaker. If you really know me you'd see that I often pair Terrakion with Mega Slowbro. I do as Terrakion is perhaps the best teammates possible to use with Mega Slowbro. This doesn't only apply to the Band Variant as the Scarf Variant works well too as it allows Terrakion to revenge kill Tornadus-T, Weavile, Mega Charizard Y, etc.
I've been preaching this for a while, but bulky offense is the best offense.