Here are a few rules of the road, a few things we've all learned, sometimes the hard way, and sometimes the right way, like you're doing by asking.
1) For between 300 to 500 an hour, you can find one or two non-porn-star ladies in each city who are total knock-outs physically, and who will fuck your brains out, giving you everything and more that a porn star will provide. On the other hand, we've all fantasized about being the stud in the porn movie banging the crap out of one of our favorite porn stars, and to make that fantasy come true for $1K to $2K can be well worth the price. Like anything else though, DO YOUR RESEARCH. Some porn stars are not very good providers. Others are amazing. Make sure you're seeing current pictures (because these ladies have bodies that go south too), make sure there are are more than several positive reviews of her on TER, and it may be a good idea to see a few non-PSEs before making the PS your first escorting experience.
2)Most of the porn stars tour to the major cities, so it's probably a good idea to wait for them to come to you. If you start planning on visiting them in another city, you might be disappointed because sometimes these girls cancel their trips at the last minute if the pre-bookings are low. OR, you might wish to go to Vegas where a lot of them reside and work. This way, if one cancels, there will always be someone else to see.
3)Keep in mind, like anything else, whether a local escort or a PS, 9 out of 10 are of average quality or below. Don't let pictures alone influence your decision to see anyone. (Especially, if a girl has just ONE picture -- That's often a possible bait-and-switch, or it's the one pic in a million where she looks good.)
4) You asked about sites being legit. Check out the site reviews here on TER.
5) Like someone else said above, to book directly, go top the PS's official site and e-mail her. Some of the girls actually answer the e-mails themselves, and they travel with a super-deluxe laptop, so they keep current. A few others have someone do it for them. Some even have a direct phone number to their cell. Just be polite and respectful in your communication with them; most will respond. Booking direct can save you usually about a third off the EXOTICA or BM fee, so instead of $1500, you'll be paying a grand. On the other hand, some guys prefer to go thru an agency, paying the extra bucks, because they say there are some advantages to that kind of booking, but you'll have to hear from them on that.
Hope this is helpful!!! HAppy hobbying!