My Resto, which I can’t log onto in my phone for some reason, has become my favorite. I enjoy the constant…. busyness of it.
I’m t honking of maining it in 11.1 but a concerned after reading these forums in regards to the mastery changes.
Will it be a decent main?
I think it’s a little too soon to give a confirmation as they are still tweaking the classes. I haven’t jumped yet on the PTR, but looking at some videos on YouTube I am hopeful 
It’s extremely fun. If you tried it out and immediately vibed with the playstyle, I’d say…give it a tier. It’s just a really good time, and I while I’ve been a healer in this game for a long, long time, Resto Druid is the most fun I’ve ever had in that role.
The mastery changes will need to be tested before we can truly know the answer to your question, and while it’s definitely an issue of concern, there really hasn’t been enough playtime with it, yet, for anyone to know what the consequences will actually be in the rankings or in how it feels to play.