1) To begin with, I made a mistake of running the SP1 installation "not" as an Administrator. The setup application didn't complain at all, but it ran for a loooong time (2+ hours!). It failed and rolled back fine, but it took too long. After I right-clicked, "Run as Administrator", it ran fine and finished after 20~30min.
2) After SP1 had been applied Visual Studio was veeery slow, even freezing at times. After some research I found that the culprit could be the Productivity Power Tools not playing well with ReSharper. I have to say that shortly before installing the SP1 I installed the latest version of the extension. After "disabling" the extension and restarting Visual Studio, everything seems to be ok, performance wise.
3) I experienced some flickering in Visual Studio after applying SP1 and noticed that the "Hardware Graphics Acceleration" checkbox was unchecked (Tools - Options - Visual Experience area). I set it back to checked, restarted VS and the flickering stopped...
Hopefully this will save yout time should you have these issues as well. Enjoy normality once again.