Are you interested in keg fermenting in general, or in serving with a top-draw system?
There are two of these available and I bought both to audition them. There is the
Clear Beer Draught System, and
this product from William's Brewing.
The CBDS is twice the price but it's also nicely engineered, heavier, and has an available screen to permit dry hopping in the serving keg. The other product is simpler and seems to work just as well most of the time. In my pre-usage tests with water, it would fail to pick up liquid in some situations, mostly due to length of the silicone tube, which could be rectified pretty easily. The tube is sized for 5 gallon kegs, and I use 2.5 and 3 gallon kegs a lot so this is a factor. Because the CBDS is heavier, the tube flexes better and I did not need to cut it to work in a smaller keg. The William's tube had to be trimmed to work properly. No big deal though.
I am currently trying these for the first time with beer. I fermented a simple pilsner recipe, 3 gallon batch, and split it into two 2.5 gallon kegs (1.5 gal each). I pitched lager yeast in one (it's still conditioning) and a Belgian yeast in the other (it's in the keezer carbing up).
I also brewed a 2.5 gallon batch of English bitter and put it into a 5 gallon keg. I just moved that one to my keezer.
So far I have nothing bad to say! I transferred from kettle to keg, pitched yeast, sealed, attached blow-off, eventually removed that and put the keg on tap. No transfers of any kind. The beer (pulled from the top) is nearly clear after 4 days, and tastes just like a green beer should. Time will tell if the stuff at the very bottom impacts flavor at all.
There are plenty of reviews about the CBDS, but very few mentions of serving from the fermenter-keg. It was one person in particular who claimed to have done it 20+ times that convinced me to go for it. Check out Angel's post in the comments on
this Brulosophy review.