I've sent off a few mail-in rebates in the past couple years. One to Western Digital, and a couple to Canon (maybe one or two others that I can't remember right now). It took a long time, but I got checks from both of them, and they all deposited without any problems. I generally don't like mail-in rebates, but once in a while (if the sum is large enough), they're worth it.
I agree, though. I'd rather just have the better price right at the check-out. Saves time, hassle, and postage fees.