If you have no other space (like me), it works well. Mine kinda hangs out the back of the stand (it's in a corner). If you can find something larger, I would recommend it. They're kinda pricey (I got mine used), and kinda small.
For a 55 gallon I wouldn't recommend this one. Just not big enough.
I ran one on my 29, and used the same on on my 55. The cheato growth once it was on the 55 was amazing, every few days I had to cut out 3/4 of it, and it'd grow right back. It did lower my nitrates alot, even though it was a small fuge. I think its so worth it, especially if you don't have a sump, which I did, but liked the hob better. I also didn't use any other light, the mh lights lit it up enough.
I have a hang on refugium for my 40 gallon. I think they are great and really cool, but be careful not to let them overflow. I have had two overflows, and one was because the lettuce algae clogged the return drain. As long as you don't let it clog and overflow, it will be great for you.
I had one on my 55 and I was always worried about it overflowing. Now that I have my 120 I have my fuge as part of my sump. I feel much better about it not to mention it's way bigger!
Tell me moe - how did it overflow - I thought most are designed to hold the cheeto in the middle and the ends of the fug.. one brings water in and the other pumps out... so how do they overflow ?
HOB fuges work with one pump, it pumps the water in, and the water goes over the first baffle. Thats where the cheato is. Then theres another baffle, and when the water goes over that baffle, it goes into the return pipe, which puts the water back into the tank. If that pipe gets cloged, the other pump doesn't stop, it will just keep on pumping the water and it will overflow onto the floor. It happened to me twice.
I had plans to do the same but it seems small, just running it with some rubble and carbon for now. Came across this link though you might want to check out.
I created that same fuge as the RC guy (Aquaclear filter), but instead of cutting acrylic pieces for the baffles I just used one of the sides of the filter basket which are already designed to fit the shape of the filter. I also changed out the impeller for an AC 30 impeller to slow down the flow. It's super-easy to build, fits in a smaller space than the CPR fuge and it seems to work great. I'm really happy with it.