Hello Shadow.
I haven't seen much discussion on the Fluval U series here. I believe Practical Fishkeeping magazine (UK) named the U series as the 'freshwater product of the year' in their magazine.
The U4 is rated at 260 gallons per hour. Here is a link to one review at Pet Solutions website:
http://www.petsolutions.com/Reviews....5510465&Mode=R
Matt Clarke from Practical Fishkeeping takes a look at the U series in an online video. Simply search via typing in: reviews for Fluval U series.
Internals don't seem to be as popular here in the US as they are in the UK.
HOB filters like the AquaClear are more popular here. However, the U series is one of the first internal filters I've considered using as a secondary filter (I like to run two filters on most all my tanks). I really like that it uses biomax and the sponge just like the AquaClear. It also offers finer particulate and chemical filtration with the poly/carbon cartridge insert. IMO, the primary drawback would be their size due to the fact that it's inside the aquarium and taking up space. Other than that, they look to be well thought out with unique features like the 3 adjustable water flow outlets, flip top lid for media changes, and being able to remove the filter without removing the bracket that attatches it to the aquarium wall.