I'm personally averse to giving up part of what they owe me to make them do their job, but 4MyRebate does offer a "No Wait" Rebate option if waiting is a problem for you. The turnaround time for online registration, mailing the documents to them, waiting for processing, and receiving the check or prepaid VISA card is around two, maybe three weeks. That might be worth sacrificing 10% of your rebate. (For example, if you're entitled to a $40 rebate, they will let you choose the "No Wait" option for "a small fee" of $4 and you'll get a $36 rebate.)
Edit: Mailing time will vary. For example, I live in the Dallas area and it only takes one day for mail to reach the rebate processing center in El Paso.