We used to have one on our old yard. At first I wasn't keenon the idea - mainly because Storm can be funny about small spaces, but she didn't mind at all and it was a god-send in the end, because there simply wasn't enough places to tie-up your horse when the weather was bad and you had to muck out. Winter turnout was extremely limited - so it was at least one way of them stretching their legs. I believe it isn't a good thing for some horses - depending on their general health. For instance, when our Joe damaged a leg, it wasn't appropriate for him to go onto the walker. However, it does depend on what you are trying to achieve recovery wise.
We also had an "incident" when somebody "forgot" to put the walker back on - after taking their horse out. Which resulted in a poor old mare being stuck in the walker in the semi-darkness - wondering what on earth her owner was playing at! Eventually, owner came back to retrieve their horse, thinking she'd have had 20 minutes walking - only to find her stood there helpless!!! Tsk. Yards eh?!