I have one on my 30. It works well, but I had to modify the drain with a gurgle buster mod (easy to make). Been using for 2 years.
These are my issues:
-Needs to be watched. If a bubble starts to form in the U, it will grow in size, reduce flow, and eventually stop. Leading to a nasty overflow situation. The fix? Clean and reprime periodically. Just a maintenance step and not too tough. I use a piece of airhose to prime it. You can either suck on the hose to draw the air out, or use an aqualifter.
-Make sure the internal box is secure. It is usually adjustable with a nylon bolt/nut or something so you can raise/lower the tank water height. If not, the box can slide and fall off. Again, a potential overflow situation.
-When doing water changes, I siphon from the display down to a bucket. The sump is too low to siphon effectively. This drops the water level in the display slightly... which leads to some turbulence in the overflow box. This sometimes leads to bubbles getting into the U. Again, reprime to keep the overflow working efficiently.