OK - thought I'd share my experience since upgrading to 2015 on my Win7 64 bit machine.
2014 worked great... no problems at all but we decided to install the 2015 upgrade. Everything worked fine for a few weeks and even after Service Pack 1. This week, I have a lag problem with my cursor. Seems to be jumpy and Autocad in general is just slower. I haven't installed anything new nor changed any settings in my setup.
Autocad 2014 still works great - no lag problem at all.
Online research showed that maybe the Aero Windows Theme could be a problem. So I went to the Basic 7 theme and my 2015 cursor is working fine again - no lag problem.
Seriously? I get an upgrade to the "better" version of Autocad and lag problems.
Thinking I may just shelve 2015 and go back to using 2014.