If you have one at home you're a bit restricted. You either set them up for walking and working and you can't run on them. There is no console or hand rails and no sign of a safety key. Running is a fantastic way to get fit and lose more weight and you can't do it when there's a treadmill desk attached, you certainly can't work while running. You're moving up and down too much to focus on something.
With many of the cheap treadmill desks we've seen you have no console or read out of your progress at all. There's nothing to tell you how long you've been walking and your distance. When I workout I like some stats to tell me how I'm doing during and afterward. Running is a natural exercise to do when you can walk briskly for a period of time. You evolve and exercise more intensively.
If you're convinced and you'd like treadmill desks for your home or office then the standard leader in treadmill desks of all sizes and types is probably Lifespan Fitness. They've been making treadmill desks for a long time and by the looks of it, that's all they do. They're rated highly by the Better Business Bureau so they should be good to deal with.
Personally treadmill desks are something I probably wouldn't recommend for the home unless you can afford a normal treadmill as well. To cut out running as part of your health and fitness is too great a sacrifice in my opinion. For the office I think they're great and an employee would enjoy working if they can get fit at the same time. It does seem strange that in our modern age we combine work with exercise but if that's what you want to do then it's up to you.