After doing an upgrade on my quite recent snow leopard installation I found the animations quite laggy, googling suggested this was a common issue for a lot of people (including 15" 2011 MBP users on the descreet cards).
I really didn't have much, if any, data to lose so decided to go for a full format and start fresh. Maybe a placebo effect but to me it definitely seems to run smoother compared to before, mostly noticable when switching the full screen apps which would always seem to lag before. I still get it sometimes but 80% of the time its smooth now. I certainly feel better knowing its a fresh install on there
Regarding the OS, I was still getting used to SL, I do prefer Lion now after having a sit down and play with it whilst installing all my stuff again. I never got used to spaces, expose etc before hand so learning something new won't be a challenge to me - overall i'm quite impressed with it, even more so given the price
Really like the full screen switching, versions and auto saving will be very handy once more applications support it and the new gestures are good, finding it quite fluid switching between a desktop, safari and mail and you can also quick peek into mail to check for new messages too by doing half a swipe