After doing rot/fester through sindra tonight I'm a little frustrated with the new spec. It's a lot of fun on trash spreading dots but impact seems like it's always up when I don't need it and never up when I do. Since they've based so much of the spec around DOTs I find it pretty frustrating that such a fundamental aoe damage ability depends on RNG.
The new LB is very frustrating. I clipped most of my casts tonight, partly just from not being used to it and partly from an unfamiliar mod setup. The spell queueing mechanic also seems different or broken making it really hard to pull off a HS pyro if you're a little laggy at the time. And HS pyro is broken in that you can't cast it while moving. All of this added up to a very frustrating night with me sitting around where I was before in DPS on the single target fights while moonkins seemed to be consistently ~6k dps higher than prepatch.
Looking at what I can change, I think my crit took too big of a dip from the spirit and molten armor changes. The mages I've seen on WoL had pyroblast as their second highest damage spell while mine was consistently LB. But with the extra hit that's now needed it's not obvious how to increase my crit percentage through gemming and reforging.
I think this will be an interesting time as more testing and more math comes together. The potential for this tree is pretty high depending on how encounters in Cata are designed, but for the fights I did tonight where there is not much AOE I'm a little disappointed.