Hey dude,
I went to UMass Amherst for 2 years before going to the Academy. I had 54 credits before I left UMass and "validated" only about 12 or 15 credits. Validation is the academy's way of transferring credits where you take a version of a final exam for the class you're trying to "transfer". It was suprisingly hard to validate courses.
Another thing to think about is sports eligibility. I burned up two years of NCAA eligibility at UMass and only had two to play with while at USNA which made me only partially useful as a varsity athlete.
The major advantage you have over kids coming straight in from high school is perspective. Most plebes spend time living vicariously through their buds at civilian school and piss, moan, and whine about how they wish they went there. While this is certainly valid, you will have already had that experience.
Hit me up with any other questions you might have. I'd be more than glad to help.