The Speck CandyShell case buttons work fine. I had a GN2 with the Seidio Active case and it was also fine. However, the OB Defender on the GN2 was a bear to work the power button. Unacceptable.
The main issue I have with the Speck case is that the back and sides are slick and it's hard to pick up when laying face down on a hard surface. The shiny back also shows scratches easily... but that's not really an issue for me. It seems to protect the phone and the camera lens very well.