They're really not that bad...
The biggest issue with people is claustrophobia... but unless you weigh like 300+ lbs, there's plenty of room! And they are watching you, so if anything happens, you can get pulled out and taken care of... it's not like you're in there all alone!
The thing(s) that bother me most are:
Noise. Granted, you have earplugs (or headphones, if you want to listen to music or a CD or something), but you can still hear plenty of banging, groaning, whirring, clicking, etc.
Pain. I have a very bad case of scoliosis, and laying flat on my back for more than 10 minutes is uncomfortable... past 20, and I'm in agony. Even with a hydrocodone pill, it's still enough to make me cry by the time I'm finished at 45 minutes. If you don't have anything wrong with your structure, I'm sure this won't be an issue... but since you do have to lay still (and they do strap you in, unlike what TV shows like House show, where they're just laying there 'freely'!), it might get your muscles a bit cramped from no movement for about an hour or so.
Boredom. Even with music or a book CD or something... it's still not very entertaining! LOL (and my music gets interrupted every couple of minutes because they have to tell me when to take a deep breath and hold it, or exhale all the way and hold it, etc. because I get chest images done)
I'm sure you'll be okay! ^_^