First time staying at the Monte Carlo and the pools were great! There are three along with a hot tub. I stayed from a Thursday to a Saturday and to my surprise on Friday, there was a DJ and it was definitely a party vibe!
The nice part was the DJ was stationed at the head the wave pool so you could go and join the rowdyness but if that wasn't your thing, there was still a space where you go for a more relaxed time.
My suggestion would be if you want to enjoy the "lazy" river, avoid peak times. There was nothing lazy about the lazy river between 11am and 4pm.
The Monte Carlo isn't the first hotel on my list to stay at in Las Vegas but it moves up a few notches because of the pool!