There are 2 different payment programs, MCFAP and MCTAP... can't remember which one is which, but it's important to pay attention to what you are signing-
One pays you actual tuition reimbursement- 7000 (tax free) per year, plus a 300/something a month kicker. The catch? A slight amount more time incurred on your contract, and oh yeah YOUR PAY ENTRY BASE DATE GETS RESET TO THE DAY YOU ARE COMMISSIONED... Big mistake, considering your pay entry base date for PLC starts the day you go to OCS the first time (meaning you will get payed as if you already had 2 or 3 years in when you get to TBS). This translates to gaining 7000 during college so that you can get 12000 less per year throughout your career. Do not take this money. As a note, if you are enlisted you will still retain your previous PEBD, so the only catch for you is the extra time on your contract. And if you are a flight contract, the extra time will no longer be applicable to you.
The other program gives you 300/something a month for the school year, you get to keep your pay entry base date, and you just incur a little more time on the contract. Not a bad way to go. Just make sure you don't sign away your early PEBD.
JBelsha is correct- if you do not take your commission you may pay the money back or go enlisted.