Three prong test:
1 - Do we have proof of payment?
2 - Is there a non-internet contact for the company?
3 - Online complaints
Results:
1 - YES
2 - YES
3 - There are some, but not enough to totally avoid the company.
1) I was first paid by E-bates on May 25, 2011. I was paid for shopping with both e-bates & Teleflora - plus there's a $5 bonus for signing up! They pay you directly through PayPal - and the minimum payout is ONLY $5 - not like many programs that insist you get $25 or $50. Here's my PayPal image.

2) E-bates is pretty good with responding to e-mails. I had to hound them to get credit for one of my referrals - but their "Customer Care" link on the bottom right of the page is great. They respond to inquires in a timely fashion, and are good about getting folks credit when it's due.
Barring that you can also find them on google maps with a mailing address and a phone number. See this LINK!
3) The BBB (Better Business Bureau) shows that E-bates has 89 complaints in the last 36 months. And there are various complaints on the web where people did not receive credit.
The trick is - keep TRACK of who you click through e-bates to, what date it was, and how much the total was. It also helps to jot down the e-bates confirmation number they show when porting you through. Many of the folks who complain online seem to think that e-bates should look up all of this information for them, instead of being prepared when they contact customer service. E-bates has thousands upon thousands, if not millions of users - so giving them all of the info up front exponentially increases your chances of getting credit for your purchases in a timely fashion. If you're not particularly good at this - E-bates has even made it easier recently - providing each user with a list of their tracking tickets!