First and foremost, if you pre-ordered you don't have to worry about getting to the register and them being out of the hardened edition - everyone who pre-ordered will have a copy reserved under their name (and powerup rewards account, if they have one); you'll have to present your receipt, and in some cases, driver license or rewards card in order to receive it. Reserved copies are held separately from those available to those who didn't pre-order, so your copy will be waiting for you.
GameStop typically sets up two separate customer lines - one for those who pre-ordered and one for those who just show up to buy the game on a first come, first served basis. Also (at least at the five midnight releases I've been to at my local GS), they assign people numbers for their places in line when they show up - kind of like being in a deli ("Now serving number ___"). Check with your GS to find out what their policy is, and you may be able to get your lottery number earlier in the evening, then just show up 30 - 45 minutes beforehand and queue up based on your number. If the employees know their stuff, they'll make sure things run fairly smoothly - given crowd size, weather, etc.
As for what it's like... well, that depends on the people who show up. Each game has its own unique fanbase, and the makeup and tenor of the crowds can be pretty varied. I've had good experiences and not so good ones. For the most part though, people are ok. It's a CoD game, so don't be surprised if it's kind of raucous and you hear a fair amount of profanity flying around; more than a little bragging going on as well. Just accept it for what it is - good natured, pent up excitement - and you'll (probably) have a good time.
Hope the info helps.
Remember, you can't have slaughter without the laughter
Message was edited by: Wolf_Odinson