I've done this 3 times so far, not with AirCombat USA, but with a local outfit called SkyFighters. They use the T34A Mentor, so the pilot is in the back.
I've gotten motion sickness other places, so I'm not immune to it, but I never got it any of the times I flew. I think it may be like driving - if you're flying, you don't get sick, but if someone else is, you might. Anyway, they gave me two of those little wristband things which have plastic buttons on them. They put pressure on a particular point on your wrist. I don't know if they worked or not, but I never even got queasy, much less sick.
I'd say skip the Dramamine, and eat what you're hungry for. If you're hungry at all.

BTW, they won't be stunned if you do puke. They're prepared for it - they have barf bags in the cockpit.

One thing they advised me was - drink lots of water so you're properly hydrated. I didn't know this, but they told me your ability to withstand G's can be cut in half if you're dehydrated.
It is without a doubt the most fun thing you can do clothed. If you've been flying long, you'll be very surprised how "at home" you feel once the combat starts. It looks just like you thought it would look, and everything works about the same. In fact, the planes are easier to control than the ones here - I never touched the trim once I set it where the pilot told me, and only touched the pedals a couple of times. I asked him about the rudder, and he said basically, use it or don't, whatever you want, it's really only useful in landing. Or something to that effect.
The G's are one thing that will be surprising. We pulled a peak of 4.5 g's my last flight, and if I hadn't known better, I'd have thought it was closer to 7. Even 3 g's feels like a ton, especially if it's sustained. I never felt like I was in danger of passing out, nor did I get tunnel vision, but one of my opponents told me he "went to sleep" for a few seconds at the bottom of a hard yo-yo.
As for stalls, in the T34A, you get a very noticeable shaking in the stick - and as I recall, the entire airplane - when you are at the edge of the flight envelope. The first time I heard it, I thought uh oh, I've broken something. It was explained to me that the plane was designed to do that when you got close to the edge, and it was basically just a warning that you were about to put more AOA on the plane than was efficient.
I stalled out twice, once during a fight (which caused me to lose that fight by going below the hard deck), and again trying to follow the other plane through an Immelman. In both cases, once I got off the controls, the plane went nose down and started flying again. The stall at the top of the Immelman was the best, because I knew that it was going to happen. As we hung there, weightless, not flying anymore, and the plane began a mild tumble back to the nose-down position, the pilot in the back said on the radio "Oh baby, ya gotta love this!". And he was right.

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Vertigo (-vert-)
CO,
The Haze