I am running a cfs-700 and its great. I believe the 700gph is for open lines so you'll get about 350gph with media installed. I have mine on a 180g FW and it works great, I usually just run the UV over the weekends which seems enough to hold back algae. I love how quiet it is, can't hear it at all, though it is pretty tall at 24" it just fits under the tank for me [with 1" to spare].
I didn't change the o-rings yet but it probably wouldn't hurt so I am going to research some more. Its a little strange not have actual baskets for the different media but there are ways to work around it and the price is very decent, I am thinking about getting a second one. I would recommend it.