In my experience, anything higher than -80dbm is good. If it's from -85dbm to -100dbm, that's considered risky and calls could drop and data speeds will be slower.
I was blown away when I switched from Sprint to Verizon. On Sprint with the BB Curve, the highest I got was -80dbm. Now with VZW I get anywhere from -80dbm up to -60dbm which I consider awesome!