I've been using Ebates since the end of 2008 so its over 1.5 years now. I've accumulated $59.35 in cash back rebates by using Ebates in that time.
Almost all of my spending has been on travel sites. I have cash back from Orbitz, Priceline, Hotwire, Hotels.com and Holiday Inn. In total I spent just under $2,300 and the cash back of $59.35 amounts to about a 2.6% rebate. The $2300 was from several hotel stays, airfare and rental car purchases over a year and a half.
I've gotten cash back on 10 separate purchases. I figure it takes maybe 20-30 seconds to go to the Ebates website and find a merchant. Even if I'm slower and take 1 minute then this amounts to about 10 minutes total work to get $59.35. Thats pretty hard to beat.
Ebates only works with online sales though and I dont' buy a ton of stuff online actually. For me it works great for my travel purchases since those are almost entirely made online.