Thicker steel is definately better, but I have seen B rate safes pried open, and those are usually 1/4" bodies with 1/2" doors.
I haven't seen the pics, but from the others on this thread it sounds like the safe was tipped over, making it infinitely easier to come to bear with your weight on a pry bar. Bolting down your safe, which is easy, is something everyone should do.
a1abdj - of the B rated safes that you have seen pried open, were they also tipped over? What kind of area were they in, and was there room to use long pry bars, etc? Any idea how long it took vs. 3 minutes on one the cheap 12-gauge safes? I'm very interested in the circumstances.