I installed Lion a little over a week ago and I'm fairly happy; it's fast and responsive for me.
The only issues I have are full-screen apps with a multi-monitor set-up, as it's the least intuitive system available, I'd much rather have full-screen apps on each monitor, with their own full-screen hidden menubar, as it'd be far more elegant. As it is I'm sticking to my previous, tried and tested, method of simply using Command-H to hide apps I'm not using, and just resizing them to give the best view.
My other niggle is with Resume; while it's great for some apps, it's an absolute pain in the arse for others, specifically ones that deal with larger files like Quicktime, which can take a long time to open if you forgot to close that 1.5gb+ movie file you had open last. Once I figured out how to choose which apps do this it's become much better. For those interested, just use Go to Folder in the Finder and open ~/Library, look for the Saved Application State folder, and simply lock it. Clear out folders for the apps you don't want to use Resume, and any time you want to add an app, just unlock the folder, open and close the app normally, then lock the folder again. Considering there is also a defaults command that can be used to set this per-app, I think it's shameful that Apple hasn't bothered to add support for per-app Resume, a simple toggle in the Dock options menu would be perfect, and dead easy for them to do.
Otherwise I'm loving it, though I can't seem to get my wireless keyboard to work with FileVault's pre-boot login screen, which I'm really hoping they'll fix soon!