Never enter an agreement that seems to be more balanced in favor of the sponsors and affiliate program than it is for the members, if the benefits and responsibilities are not balanced you could get left holding the bag.  Contact the affiliate directly before signing on with a simple question, if you do not receive a reply you can determine that you will not receive service from them after you've signed on.

You are going to have to look at two very separate set of terms, one with the affiliate program, the other is the terms of commission by each sponsor... read both carefully!  What looks like it could be a potential gold mine might be anything but, read over the commission structure completely, then examine the sponsors page for anything that might be contrary to the information posted with the affiliate.  Sponsors that run different promotions with a variety of affiliate programs that pay differently according to affiliation should be reported immediately since all offers should be consistent regardless of affiliate.

A little knowledge can go a long way with such programs, and digging deeper might reveal that the sponsor is on the level with the affiliate program and members, but it's offer is not all that for the customer.  Read the fine print, if your sponsor is offerring little more than spam, and is collecting and reselling demographic information on your guests, it can have a complete negative impact on your page attendance and reputation.  It doesn't hurt to be selective, even if the offer is for your personal favorite product, where there is a catch involved it will most likely backfire on you.