MEPS blows. I went to the MEPS in phoenix (same building as my NRD) and I was far enough away that they put me up in a hotel (a NICE room, and by myself). I was downstairs by 0500 and probably didn't get a ride back to my motorcycle until 1300 or 1400. Bottom line, take the day off. The things that were time consuming for me were: BP was borderline high due to lack of sleep and nerves so I had to come back for a retake. Waiting for the semi-present sgt to watch us piss (and the countless dudes chugging water and jumping to coax a shy bladder). CONSTANTLY having to go back to the eye exam section since they kept leaving out tests required for a commission since they didn't realize I was an OCS candidate and just general confusion about where I was supposed to be/cattle scenario.
I probably had to say 100 times that day that I was an OFFICER candidate and that I needed certain tests they left out. My advice is to be proactive. I asked if I was done a few times and sure enough, they said "oh, officer? You need to go back for another eye test. They left such and such out...". If you want to go there once, make sure you get everything done you need done.
The staff WILL be curt and pushy to the point of being borderline rude. This is (allegedly) due to the fact that they are trying to push these recruits through quickly so as to limit the thought process of where they are going to prevent too many from backing out (hence the early wakeup, rushed exams, and swearing in the same day, ship out the same day or next).
I was overly paranoid but I didn't really bring that much. The issue I needed a waiver for was handled before (or so I thought) and my recruiter was in the same building so I didn't think I needed to worry. I brought US passport (citizenship proof...dual citizen), other ID, my medical pre-screen and I think that was it. If you have waiver issues like PRK or stuff with associated paperwork that you have, bring it. Better safe then sorry. You do NOT want to go back. It sucks. Bring a book (if it doesn't fvck up your vision) or something to keep you entertained and get some sleep! I didn't and it made my day worse (plus the 5 hour motorcycle ride back to CA after....).
Best of luck. MEPS sucks, just like some other things in this process, but it's just another hurdle you jump to get closer to your commission and your future career.