Now that we've had a chance on showdown to test these megas in custom games, I've gotten about four matches in with a stall team around mega slowbro... To be honest, it's a bit underwhelming compared to what we expected. The fact is, there's a ton of special electric pokes flying around and the weaknesses still exist. The difference this time is the fact that you have to recover while you're in instead of just maintaining momentum and regenerating. This really matters when you're hitting off against Landorus and other U-turners who still give you good chip damage but now you don't recover that by switching out. That being said, he's a fantastically solid wall. It's just there is a lot of SE attacks around to take him on.
I think Thunderbolt is going to be required on him, though. Water walls are taking him too lightly on the special side, especially other (non-mega) slowbros and rotom-wash. I haven't gotten around to testing the CM set yet, though.