My dd was breech at 37 weeks, then continually flipped herself back and forth for every check up, finally settling cephalic for my due dates, only to be breech again when I was two weeks overdue and due for induction.
I had ECV the day they found out she was breech. First I/we were monitored for half an hour to make sure we had a good trace and there were no potential problems, then I was given an injection of drugs to relax my bump, they made me feel a little shaky, slightly nauseaus and not quite on the planet, but nothing too awful. Waited a short while for them to work, then the consultant did a mobile scan to check dd's position and that of the umbilical cord etc before grabbing the top and bottom of my bump and spinning her around in one movement. She turned first time, no problem. Then they did another quick scan to check all was ok with her and monitored us for an hour to make sure she was settled and not in any distress. I then spent the whole of that night sat bolt upright to stop her turning herself back before I was induced the next morning. 
When he turned her, I would say it was uncomfortable, definitely ouchy, but not agony by any means. Personally I felt more sick and it was more painful when she turned herself towards the end of the pregnancy.
Iirc, the consultant said they would only try twice and if she didn't turn within a couple of minutes they wouldn't try again.
I think it depends on the type of breech presentation as to whether you can try for a vaginal delivery. With some its possible, with others more risky. In my pct they didn't have even one midwife who had attended a v-breech birth, so their policy was not to allow it, as their wasn't enough professional experience. Its something people have pretty strong views on to be honest.