I have installed Yosemite on my 27" iMac, the download weighed in at over 5GB and took quite a while to download even on my 100mbps BB. It is release day so to be expected. The install didn't take long then then was a raft of updates.
Machines will run a bit slow after install while it all sorted in the background for Spotlight etc.
What do I think? its OK, it will take some time to get used to but it does look good and I am a fan of the flattened look even though I had some trpidations about it before hand. Notifications are good and the email App seems to be better, iTunes looks good. I am still getting to grips with it will post back later.