I tried to get one of those and it was was unsuccessful. I went at 29 weeks and again at 32 weeks and both times the baby just happened to not be in the right position supposedly. Either that or the ultrasound tech just wasn't good at her job, but it was a different person each time.
I would say 28-33 weeks would be good for best results
I got mine at 28 weeks n 5 days.