Did FL120 once in a Pa28-161. Bit mushy up there. Found it setled at about 80kts. The countryside looks a bit different so be sure fix your position over the ground with vor or gps if you can. Lean off on the way up every 2000' for best performance. Keep the cylinders warm on the way down and don't forget to move the mixture knob forward again
.
For added fun and a great view try it on a clear night, stunning (especially the first time).