I think it all depends on where you go and whether the baby is in the mood to be looked at!!!!
We went for one when I was about 25 weeks with my last LO. We went to a rather dubious place called 'window to the womb' in Nottingham. It was a bit of an odd set up as the place was actually someones house who lived there, and the whole family were sat in the conservatory!!! Then baby had 'EVERYTHING' she possibly could in front of her face, I mean her arms, hands and even legs, so we didn't get a good shot of her at all and the one and only glimpse we got she looked like she had Mike jagger lips and the biggest nose I've ever seen. I came away quite worried!!! Anyway we asked to be rescanned due to poor pictures and they reluctantly obliged. But it really wasn't the best experience ever and certainly wouldn't go back there!!!!