a lot of people use them to save money over canister filters... people also believe that the filtration is better because you can customize it more so then with something like a Fx5 .. on the flip side canister filters seem to be easier and less work in the long run
i use Fx5's on my 150 and 125 along with rena 3's but im going to switch to sump so i can hide the heaters and UV under the tank in the sump ... mostly i think it comes down to what you really want .. read up on both and check how much you want to spend and that should help