I had one done @ 30 weeks, and was brilliant well worth the money! I went to one in Warrington, it was great, not rushed, spent a long time scanning me, and the dvd was brilliant.
I saw my ds smile, rub his eyes, and yawning, i got a CD rom with loads of piccies on, the DVD and photo keyrings.
Ther are a couple of piccies in my profile if you want to take a peek, i saw some pics from scans taken earlier on and they wasn't as good i don't think, i think the later you have the scan done the better really, i think the cut off point is 30 weeks though as after that they just get too big and squashed up to get decent pics.