i'm using a united 48". happy with the build quality, and the size seems appropriate. i would certainly recommend it based on my experience.
it's nice and heavy. i haven't had a need to add any weight to it for my applications so far (mainly stripping turf and creating swales to divert storm water). it's also GREAT for moving soil, easier and higher capacity than the FEL. downside of course is that it can do some damage to the turf you're dragging over, but surprisingly little IMHO. this box allows for all four scarifiers to be positioned upside-down for transport or hauling soil, another feature that i can remember not being available on all models that i compared.
i can remember while shopping for it that some of the SCUT-specific models are pretty light and a few users talked of having to add weights to get them to dig in properly.
FWIW the owner's manual for the BX specifies max width of 48" and max weight of 375# for box blades. mine weighs ~320#.
i'd be hesitant to drag anything wider. with a good load or the scarifiers set aggressively this thing will stop my BX dead.
United 448HD Heavy Duty Tractor Box Blade 48"