The cooling effects of evaporation are significant. I used 2 cheap clip on fans (6” in diameter I think) to cool my 6’ 150 gal reef (about 200 gal system total). I used a controller to keep them temp at 78.5 degrees during the summer. The tank would get to over 83 degrees without them.
Of course it significantly added to evaporation, but it’s a non issue with an auto top off.
It looks cool, the evaporation has a significant cooling effect as already mentioned, but you have to keep an eye on the humidity of the room that the tank is in, and the water level of the tank.