I had one of these a couple of weeks ago. Its absolutely fine, and exactly as GNers have said above.
The only thing was they said to keep as still as possible and I kept needing to swallow (a bit like when you have a dentist doing stuff in your mouth), and I didn't know if this would count as moving. I did swallow and the images came out fine ~ but it might be a question you could ask before you go in.
Also, after I had finished and I needed to walk to another part of the hospital, my balance was a bit out-of-sorts for a short while, but I think this is more to do with other stuff and nothing to do with the MRI itself.
It really is absolutely fine and lasted about 15-20 mins I think