My boyfriend has one in his Fluval Roma 125l replacing his old Fluval 3+
He thinks it's excellent as it's so much easier to rinse out the sponges with how the top lifts up so there's no need to remove the whole filter from the bracket
Also the 3 different flow outputs is a nice touch, especially the spray bar middle one
He used to have a problem with the impeller on the old 3+ when after he'd cleaned the sponges and what not the filter wouldn't start and would need the impeller taking out or just fiddling with to get it working again, but with the U4 it is at the bottom of the filter, enclosed I believe and he hasn't had that problem once
All in all he thinks it's a great filter and would recommend it
I have two which i brought around 3-4weeks ago, they seem a good filter and iam a fan of fluval, I have just set up my new tank which i just placed the filter into the tank,which has matured in the five footer,i tried also to set up my eheim 2213 which id been waiting for a part, got part put it in, filled filter bla bla, then suddenly rattled,took apart and the new part i didnt push in enough, so i pushed again and broke the new part so got to order another one...lucky i have these filters spare..