Not a scam, per se, but not really a valuable offer either. Basically, in order to get the gift card (and my offer was only $500), you have to sign up for a bunch of other programs, the majority of which have upfront payments or monthly fees associated with them. You also need to refer other people to join, which is really how they get -- most users usually can't meet the referral requirements. So offers like this are generally a waste of time.
When I have a chance to get around to it, I'll put together a hub explaining how this works in detail.