I've been using the TR1200-DT3 for about 1.5 months, and I absolutely love it. It's built very, very strong and is extremely quiet. I bought this version of the treadmill because I have a built in desk that doesn't move and because I have four monitors--so I can't simply plop a laptop or single monitor up on a desk such as the one that comes with the TR1200-DT5. Also, I want the option to sit when I want to, because I don't think I'll ever be walking 8-10 hours per day (and, in fact, these treadmills only support 6 hours a day).
The treadmill really is quiet. I often participate in conference calls and I use a Polycom speakerphone right from my 11' x 13' home office. No one on the call can hear that I'm walking. The motor itself is virtually silent and the only sound you hear is the belt slipping along the treadmill track (very quiet) and your feet hitting the treadmill. Because you'll be walking slowly, your feet don't make much noise at all (or at least mine don't). I walk barefoot on mine.
Also, it's easy to move. There are wheels on the front end, and all I do when I want to switch between sitting/walking is lift up the back end a bit and roll the treadmill away from my sitting area. I don't even unplug it, so it's easy to do.
I'm currently walking at 1.3 MPH. This may not seem like much, but for me it's a sweet spot because I have to do a lot of thinking for my job and a lot of typing, and this is a comfortable speed. The thing about a treadmill desk is that it's not really about speed--I get speed workouts in when I run and therefore tax my cardiovascular system heavily at that time--it's really about moving your body throughout the day. Walking at a slow pace accomplishes exactly what a treadmill desk is for--getting your body moving. There are a lot of muscles involved in walking at a slow pace. These muscles are all over, too: your feet, calves, thighs, hamstrings, groin, hips, glutes (your butt), obliques...honestly, all the way up to your head. Pain that I used to experience while standing up, say, in a long checkout line has vanished. I've got a pretty strong back but for some reason standing up in a still position would really bother me after even five minutes (and I'd be forced to stretch), and now that pain is gone.
Another thing that I notice in my situation is because I work in the downstairs portion of my home, which tends to be cooler than upstairs. During the colder parts of the year (I'm in the Boston area) I would have to wear a heavy shirt and heavy socks to stay warm, and even then I'd be a little on the chilly side. While walking on the treadmill I wear nothing but a t-shirt and a pair of shorts, with bare feet, and I'm plenty warm.
The biggest hurdle for me was that I have four monitors and a built in desk, and though I was hopeful it wouldn't be difficult, while walking my monitors were a little bit too low for me. This became annoying after walking for a while. I had to stare strongly downward at my main monitor and even my upper monitors were below my line of straight-vision. What I ended up doing was building a monitor stand that can raise/lower at the press of a button. I wrote a detailed blog post about it here, for those that are interested: http://steveseymourblog.blogspot.com/2012/11/BuildingTheUltimateTreadmillDeskPartOne.html#!/2012/11/BuildingTheUltimateTreadmillDeskPartOne.html. It's a seven part blog post, so hang in there if you think my situation applies to you. I hope my posts inspire someone to go for it, because now I truly love my treadmill desk configuration. Honestly, it's not that hard to do even if you don't have a ton of carpentry skills. I purposefully kept the building of mine simple with the hopes of leading someone else to a great treadmill desk configuration.