Ok, tried the new 5.0.
O M G
I love it, every bit and piece of it, I even added clique and totally ditched grid. Theres only like 3 things that I would love to see, and I'd be totally happy with everything. (Btw, you weren't kidding about the healthrace bar. Its godly in pve as a MT, but zomg jawesome in pvp)
Now i'm not sure how difficult these things will be to implement, i'm thinking more as possible plugins, rather than de-facto, since not everyone will need/use them.
1) aggro warnings on the raid frames. This is hands down one of the primary reasons i used grid/clique. Especially for Sarth+3 it just got to be so easy to see the red frame around someone who wasn't in the tanks group, and click to taunt off them. (pally taunt FTMFW)
2) (smart) Decurse warning. Just some indicator on the raid frame if someone has a poison/curse/disease/magic effect that your class is able to remove. (Again combined with clique is amazing for not having to worry about yet another mod like decursive. just means i gotta pay more attention)
3) Infopanel plugins are t3h borken. I half expected this, but also half hoped it would still work. the big ones i noticed, as they're the only ones i worry about, are the recount/omen ones. Especially the combined dual-stats one. 
EDIT: After some monkeying around, it would indeed appear, these are working. HUZZAH.
Otherwise, after some mild stress testing in numerous bg's, and some more testing tonight in 10-mans. I hope to be able to say without doubt that since being able to drop a few addons, and the new design in nui, i'm stable as ever, and improved in fps even if just a little.
Outstanding job Spiel, simply outstanding.