I just started playing one. I wanted to make a water based monk. Came THIS CLOSE to picking a Water Genasi, and wound up with a Human Variant with Magic Initiative, taking Shape Water, Frostbite and Ice Knife. I wanted a little more of the elemental flavor than just 4 elements gives, but I also wanted it to be a little more combative than what the Water Genasi would have given, though I do regret it now cause with Water Genasi, I could have breathed under water, but I don't know if that'll ever actually come up, and even then, I can use Shape Water to hold an air bubble around my head if I need to. That'll last a while, at least.
Anyway, I've found it fun. He's definitely a monk first, he just has other options that a normal monk doesn't have. He came in late to the game, so he started at level 6, so he has Water Whip and the other water based one, where you can reshape the terrain, and Water Whip does come in handy when you want to try to push someone away and deal some decent damage to them. He's only got six Ki points, and Flurry of Blows often seems like the best bet, but on hindsight, with a little more use of his water whip, he could have controlled the battlefield a little more that would have made the difference in some of the fights. Now that I've been ruminating, I think that would be the best use of his 'magic', as a controller rather than just a hitter. It makes him a primary warrior class (monk) with some controller capabilities, such as using water or wind to push people around and manipulate the battlefield.
But he was fun to play. I've never played a warrior class before, and I must admit, I liked rushing in to the fire giant and slamming my quarterstaff into the back of his knees, then darting away out of his reach.