Doesn't hurt too bad. Of course it's a needle going throw skin but if you can tolerate being nicked by a razor than you'll be fine. However, I must warn you that this type of piercing does have long term effect on teeth and gums especially if the inside is not flat. Speak with your artist.
my brother is a piercer and a tattoo artist and he he said don't ever get a monroe cause one of his friend's teeth got really messed up and she had to pay 3000 bucks to get her teeth fixed. that was enough for me not to do it, but if you wanna do it then do it and if the piercing touches your teeth or gums then remove the piercing.