I'm sure you'll get lots of responses and advice, but I'll chip in. I replaced the stock tire at 4300 or so, as it was pretty much bald.
I put on the Mefo Explorer, which now has about 3400 on it, and it still has probably over half the tread left. It feels pretty grippy, but I haven't ridden it much in the rain.. too many idiots that don't know how to drive cars in the rain around here.
I try to run it at about 32-35 psi, and know that you will get uneven wear of the tread knobs if you run it at low pressure on pavement.
So far, I'm really pleased with my choice.