So, when the 5k iMac came out, everybody hated the underpowered GPU. Now that people have used it, how is it? I'd love a 5k iMac, but I'm afraid that with the number of windows I keep open at once (20+, ~30% of which are PDFs), the 5k iMac will lag. I've heard people complain about windowing issues. How is it really? Do you see lag/jittery behavior? I really want to get rid of my Mac Pro and just use a 5k iMac, but I can't deal with lag. Also, anybody run another 4k/5k monitor or a thunderbolt ACD with it? How is it? Thanks!