I use a 12" aquafuge, with 13 watts of light, 3-4" of sand, a constantly growing wad of chaetomorpha and happy cultures of mysis, amphipods and bristleworms.
Overall, it's fine. It's easier to set up than having to plumb a separate tank. From my point of view, a fuge does 3 things.
1. Nutrient export.
2. Culture of small, tasty creatures.
3. pH stabilization when run on a reverse photoperiod.
The Aquafuge does 1 and 3 extremely well. Lots of pods and the pH varies little from day to night.
Nutrient export is harder to measure. The chaetomorpha is growing fast, and I remove about a handful a month, which is decent export I guess.
The rio is for pumping water from the display to the fuge. I hate having powerheads in the tank, so I use a closed loop which pulls water from the tank to an external pump. Part of the return goes to the fuge.