I have been using one for about a year. It works great. The top opening in my fermenter is about ~ 7.5" Dia. The big opening allows for access inside for cleaning. The lid comes with a removable seal for easy cleaning.
Now, there are a few thing you need to keep in mind. First, this is a "can". What I mean is that inside you will have folded seem joins. One on the side wall and along the bottom. You need to pay extra attention when cleaning. In my case, I use a steam generator to do the cleaning a sterilization (I also use Star-San for a final rinse!) so the joins are "no factor".
Second, the valve that came with my fermenter was not the best. I replace it with a regular 3/8" ball valve. This is were it gets a tricky. The thread on my fermenter is a straight thread not an NPT thread. I think it is also call "BSPT". I found a coupling at McMaster Carr:
P/N - 4810T223BSPT
Low-Pressure 304 SS Thrd Pipe Fitting 3/8" BSPT Male X 3/8" NPT Male, Hex Nipple, 150 PSIIn stock at $6.81 Each
http://www.mcmaster.com/#cad-2d/=65ilsu
Hope you find this info useful!
Cheers!!!