I've only been to 1 prerelease so far, but I can tell something about it I guess... (in the Netherlands, as you all probably know by now xP)
You get 8 boosters for a reduced price, that was the main reason I went. xP I'm poor, lol.
You also have to think a whole lot different when deckbuilding. With that, you have limited time to do so, we only got 45 minutes. Cards that usually are awesome(mainly stage 2s) suck at prerelease('cept for Infernape, you're god if you managed to draw a whole line of that) and vice-versa. You just have to build a fast-hitting deck with the least types possible and play a LOT of energies, since you probably don't have a draw engine and about 6 trainers total. Btw, I ended 2nd, only beaten by our Nats champion, so I was pretty proud of myself ^^ it's a very different experience than normal tournaments tho, and well worth to check it out. It also gives people who don't have 3 Flygon ex d or just can't build the deck of their dreams yet because of a lack of cards (like me) a good chance because everyone starts a deck from scratch with only a few sucky cards to choose from (for example, the best cards I got were 2 Rhydon and 2 Wobbuffet, so I went and built my deck with those).
And it's likely there will also be side-events and limited edition goodies for everyone(a pack of 60 D/P sleeves, in this case) , and of course a cool prerelease card.
EDIT: I see it hasn't been mentioned before, but the deck you must build is only 40 cards.