I always send them in, it's nice to get a few dollars in the mail long after you've forgotten that it was coming. Of course, I'm talking about $25 or more on something like RAM or a graphics card, a $2 MIR on a $10 item isn't worth the hassle. The 'scam' of rebates is the fact that manufactures know that only about 20-30% of people are going to send them in time, others will buy an item because it has the rebate and then forget about it or do it wrong.
I've had good luck recently with Corsair and Asus, the Asus one came alot faster and was a check that I could deposit. Corsair sent pre-paid credit cards.