I had one a few months ago and although I am not claustrophobic, it was pretty close. I was in for about 20 minutes, and, as someone said, it was for my back, so my lower legs and head weren't in the tube.
I did the same as Miep: shut my eyes and just let my mind wander. Normally I can doze off quite easily, but unfortunately I didn't in that case. Probably becaue of the clonkings. There was music, but I wasn't asked what I wanted.
Just as you think you can't stand much more, it's over and you can come out again.
A colleague of mine tells me she didn't like it at all and got them to stop after about 5 minutes.
(There is an alarm button they put into your hand just in case)