I used to use the Fluval U2 on my Juwel Roma 90 litre tank.
Its a perfectly good little internal filter.
Has 3 flow outputs as well as a Venturi output (for extra surface agitation and oxygen into tank water)
Inside there are 2 compartments for your sponge media as as well as carbon to slot in on top of sponge but personally I never used the carbon, I just added a bit more sponge media.
Central Compartments holds the ceramic rings media.
All these compartments are in plastic and very easily pulled out of the filter.
Access could not be easier, has 4 fairly strong sucker on which you attach the filter holder onto tank glass wall and then slot in your filter. Filter can tip forward and be easily pulled out for access to compartments for maintenance cleans. Then simply push back toward tank wall until clicks back into position.
Access to the impeller was easy as well for maintenance cleans.
I think, for the money, its a very good little filter and have no complaints about how it works, does exactly what it says on the tin really!
My only reason for swapping this filter for an external is the fact its quite bulky and takes up some space inside the tank. Thats all!
I would recommend this filter to be honest.