As far as what to watch out for? Yes, there are some things to watch out for, such as being sure there are no fees involved when one signs up for these types of sweepstakes offers. Though "Opinion Direct" did not, some sites might also also require you to sign up for other similar sites, which doesn't appeal to me personally.
Then, too, these sites also offer services for several different types of advertisers, so at some point, more often than not, you will have to filter through "yes" or "no" offers asking whether you would like more information from any specific advertisers, aside from any opinions you might have. You are not required to accept any of the offers in order to enter the sweepstakes, however, be sure to check all of the answer buttons, to make sure you are saying "no" to anything you do not want to receive.
On the other side, you might be pleasantly surprised in finding something new that truly interests you (I found some things on investing in gold, which I found very interesting, although I've never done it). I found these pages very easy to get through, though some might not. Some of these pages will also have a "skip" button on them that you can click on if you wish to pass on all the offers, so don't be afraid to decline if that is what you wish to do. Also be sure to pay attention to what you are answering, as opposed to rushing to get through to the end.
Also, as always, be sure to read through the official sweepstakes rules. In the official sweepstakes rules of "Opinion Direct", the site allows one entry per sweepstakes/per email address/per day, regardless of the method of entry (The rules also show that this site has several sweepstakes on an ongoing basis, each paid out quarterly... when one is over the next one begins). So, to enter the two sweepstakes that I've mentioned above, two surveys would suffice on any given day in order to receive the allowed amount of entries. In other words, about 12 to 16 minutes.
Though this may be more time than it takes to enter some other sweepstakes, the chances of increased odds could easily make it worth the time (which really isn't all that much). In taking the time to enter this way about once or twice a week, you would have somewhere between twelve and twenty-four entries in any given quarterly sweepstakes, and you would also have given a heck of a lot of opinions!
Finally, you also should know that in many cases there may be several sites offering different creative venues for what is ultimately the same sweepstakes. This is done for a variety of reasons, however, don't let it scare you. It may not change the entry pool by as much as one might think it could. Some people just don't enter sweepstakes this way.
All-in-all, though these types of sweepstakes may not be for everyone, and could take some time to enter, anyone who is avid in entering sweepstakes may enjoy giving them a run-through, and potentially adding them to their sweepstakes regimen. I found "Opinion Direct" to be relatively quick and easy, and without overly burdensome requirements to enter.