I tried it a couple years ago and this is what I found.
I only liked it for casual gaming. Anything more and either my eyes would strain or I couldn't play as well as I did at a desk. Not all PC games have text that is large enough to read from far away on a HD tv. And FPS games like battlefield I felt my play style wasn't as good lounging on the couch.
The mouse/keyboard issue. I prefer wired mouse/keyboards so I had to run an extension USB cable to each device which was kind of fugly. Also finding something to comfortably place the keyboard and mouse on to use was a pain in the but. And overall still wasn't enough space to properly use my mouse while gaming.
In the end I went back to a desk set up to say the least.
Before I was into console like PS1 and PS2 until I got into Pc because of COD4. I love first person shooter and Online playing. When PS3 came I tried playing it but somehow my hands got accustom to mouse and keypad. Eversince then my PS3 has been left in the corner. Though I seldom play it but I prefer sitting on my desk with my PC
She is in the kitchen making the snacks... I refer as wife, she doesn't like "cup holder" very much
my only suggestion is if you do have a large uniform color wall to consider a projector. They are about as expensive as large TVs but in my opinion they get a much better image quality of a PC screen than an HDTV does.
The only down side is you have to yell at everybody who tries to walk in front of it.
Stay at the desk, I tried the transition a couple of years bag and it just didn't feel right, couches are for watching movie's and desks are for gaming. Also, that's a sweet set up Garethedwards I am mega jealous.