I really hate rebates. I just spent an hour filling out and collecting the various required pieces for two $50 rebates from Alltel...which I still technically have not completed, because I still need the wife to make some required copies tomorrow at work. ...then I will hopefully get my rebates in 10-12 weeks and they will be in the form of those fucking annoying Visa gift cards that work about half the places I would like to use them. On the other hand I had a wonderful experience redeeming several hundred dollars worth of rebates online with Apple, taking approximately 3 minutes, and the check arriving in 10 days.
The wikipedia article contains a lot of good information on the rationale behind rebates and why companies use them (other than just because they hate us).
Wikipedia Article on Rebates