I had one done at 26+5. Mine was good and although baby slept through the whole thing and wouldnt wake up it was great to see its little face, and see it has already adopted my OH's favourite sleeping position (arms up behind head).
We were recommended by our hospital to do it between 26 & 32 weeks as there is still lots of room for baby to move so you can get good pictures.
Our friend also had one done when they were about 28 weeks and theirs wasnt very good at all, they couldnt get clear pictures of baby's face and ended up going back for second scan another day and baby still wasnt in a good position so for them was a waste.
I think it all depends on what position your baby is in.
We only had it done though as my parents live in NZ and wont be here for the birth but were here for a few weeks so this was something special for them (think I even saw a wee tear in my dad's eye - very special indeed), otherwise my OH said he couldnt justify paying for it.
We had 2 done there, an early pregnancy scan and a gender scan. They were excellent and very friendly both times and the sonographer was the same lady that does the NHS scans at the hospital during the week.
I had one at 28 wks - great photos (the lady really tried - baby was not in a great position but the dvd wasn't that good and then it didn't work on our computer so they said to come back again for a free scan - this time dvd was great and we got some extra pics for free, it was the same sonographer and she really took her time to get the best possible view, was very patient and waited half an hour while I walked around eating chocolates to help baby "move"
Oooooooooooooooo thanks for the replies. I am excited now. Would have loved to have one a bit earlier as i know at 34 weeks we might not get good pictures but this was bought for us as a wedding anniversary gift which is this weekend or i wouldn't be able to have one at all as they are a bit pricey. So excited to see our little bub again and have someone nice scan me!! Had an emergency growth scan on the NHS at 28 weeks as midwife sent us through after measuring 4 weeks behind and the lady that did it was awful. We were there for 2 mins and she just said "There is head, body, legs. slighty above average size, don't know why you have been sent here". So over all left feeling silly. xxx