Today, anyone could be that guy. Photographer: David Paul Morris/Bloomberg

Google is briefly opening the doors to its Google Glass explorers program on Tuesday, making it possible for any U.S. adult with a shipping address to become one its early testers for its smart headset. Sign-ups start on the Glass Web page at 9 a.m., Eastern.

Before you rush over to the Web site to sign up for Glass, however, here are five things you should know.

1. You will need $1,500. You're still trying to get into Google's early testing program, which means that you'll have to pay the full developers' price of $1,500. Chances are, that price will come down whenever Google puts Glass on the commercial market, but if you want access now you will have to pay a pretty penny for it.

Google is expected to make the product available to the mass market at some point, but hasn't given any concrete indication as to when that will be.

2. Spots are very limited. Google hasn't said exactly how many units it's selling today, but the company has seen a lot of interest in its invitation-only Explorer program and chances are its site will be a virtual madhouse. (Luckily, it's all online, so there aren't any insane lines.)

3. If you get one, people may hate you. Current Glass explorers have been attacked, mugged and harassed for wearing their Glass headsets in public. One man was detained at a movie theater, and one woman had Glass actually snatched off of her face in a San Francisco bar.

Google stresses that this is not the typical experience for Glass users. But concerns about privacy and surveillance are common topics of conversation for people wearing Glass out and about. Be prepared for that.

4. Yes, you can return it. If you're lucky enough to get your hands one one of the sets going up for sale today, maybe returns aren't at the top of your mind. But, if you change your mind for some reason, Google does have a 30-day return policy for Glass -- just as long as you haven't done any damage to the unit at all.

5. It's not going to be an perfect experience. Don't get us wrong: Google's doing cool things, and Glass is an interesting experiment.

But this isn't going to be a completely smooth experience out-of-the-box. You're still buying something that is, for all intents and purposes, a developers' prototype. It's for the company to learn all about the good and the bad of the product, to find bugs and to learn what Glass is lacking.

There's also the question of apps. Developers have been working for over a year to make cool apps for Glass, and there are plenty to use. But if there's something you really want,  you should also realize that you may have to just build it yourself.