For me Yosemite has been very fast and fluid.
Mail has had problems with Exchange but under the latest .1 beta it's a lot better, and I am also running the new Outlook in conjunction. Under .1 beta the stock mail app is only slightly slower in getting new emails than Outlook with Exchange on an O365 business account.
Most WIFI issues also seem to be fixed. I have not had any problems getting or maintaining a connection but reading around a few forums the beta is fixing a lot people's problems. Don't know the .0 releases always have so many WIFI issues but .1 should be out soon for those with problems.
And yeah Yosemite looks like a RAM hog on paper but for most people it's a faster OS. I got a ton of beach balls all over Mavericks and with Yosemite its much more rare. If you use something like iStat you can see the actual active memory isn't that high. I have 6 apps open right now, and my active memory is only around 40% on an 8GB system.