I use 5dimes. I'm in the U.S.
Have never had issues getting paid out. And 5dimes has prop bets and live betting for MMA too.
The only thing that sucks is that 5dimes and Nick Kalikas split ways. He is the clear cut #1 guy for setting lines for MMA fights, so 5dimes used to have their prop bets up really early and would open lines early too. They still open lines for some fights early, but only after some other sites have posted theirs. And now the prop bets aren't up until a couple days before the event.
Live betting between rounds is great though. There's Euro sites (can't access in U.S.) that do real time live betting which is a whole other level, but even betting between rounds can be profitable. Every now and then they will put a line up that makes ZERO sense between rounds where you can make a lot of $. The issue is if you're too successful they limit you and then you can barely bet anything (I was limited to $20 live bets awhile back--not anymore but for a bit I was when I was having a huge year).
I would highly advise that if you are gonna do some MMA betting, visit the MMA betting forum on this site. We focus on the betting aspect of MMA, and there's a TON of good info and dialogue about betting on MMA.