I had a gender scan at 16 weeks with babybond, it's the earliest they do it but the sonographer said that unless they were sure of the sex they wouldn't announce it and would ask you to go back (for free), it's more than 99% accurate and in all their time they had only ever had to ask one lady to go back and had a 100% success rate at that clinic (Stoke-on-Trent).
They take time to get shots from below looking upwards so they can clearly see the genitals so nothing can be tucked away and show you your pictures compared to a boy/girl, ours was identical to the girls.
I paid £79 for it and they did both normal scanning and 4D, they also printed out 6 pictures in both formats and put about 15 on a disk, I prefer the normal scans though, I find the 4D ones a bit creepy.
I had 4D scans at 12 and 22 weeks. At 12 I was given a 70% certainty as to the gender, and this was confirmed as correct at the 22 week scan. I agree that the 12 week ones were a little creepy, as the baby is a bit bug eyed at that point. At the later scan the pictures were very clear. You are able to see exactly what the baby's face looks like. One midwife told me she didn't like them, as it made it so much harder for the parent if something goes wrong. I guess that is a consideration.