I'm a little leery about using them for event horses as I think "feel" is a more important tool for conditioning sport horses but they do force one to use objective criteria, which can be helpful in knowing where you really stand.
We've got a treadmill and a walker.
The treadmill is a sloped version - only used in walk. To get the harness horses started we find it invaluable as sometimes we can't work in the field if it's wet. As with anything we don't use it as the sole fittening method but through the summer the harness horses do about 15-20 minutes in the morning on the treadmill then we drive them in the afternoon for about 20-30 minutes, and the others go on through the winter to have a leg stretch if the ground's too bad outside.
Minor problem with the TB's though - one of them is too long for it so she doesn't go on, but then she's 17.2 (and the reason we now have a walker).
I don't because couldn't afford one and have plenty of hills anyway around us- but if we didn't have hills or weren't able to hack out (and i had more money) i reckon they would be worth the investment as a good fittening aid- i've seen them in use at 2 top dressage riders yards- one was a flat one and the other rider had a sloped one to mimic hill work.
And as someone else has said a lot of racing yards have them as well.
(I think i'd prefer my horses to go on a treadmill rather than a walker as they wouldn't be on a constant turn on a circle).
Laura B uses both a treadmill and a water treadmill for her horses, according to blogs/interviews on Horse Hero.
I think a lot of the very top riders use a treadmill instead of a horse walker, at least as far as dressage is concerned. Much better for the horses than a walker.
I find the list of Horse Gym 2000 clients quite impressive:
One of the big advantages for dressage riders is that a horse on a treadmill is forced to maintain a very regular rhythm in walk, while in a walker horses can slow down, then accelerate or even do a few strides of trot to catch up, etc. -- not ideal at all.
Treadmills can be useful for recovery from injury as well, especially tendon as the horse can walk on a level even surface and the speed can be controlled and the level of exertion gradually increased.
Think they are more beneficial for a horse than a horsewalker, but of course you can only put one on at a time.
By the way, they aren't a new idea either. An eventing facility in Ontario (not exactly the centre of the sport horse universe ) I rode at had one that dated from 30+ odd years ago. It was built in the bottom of a concrete "dip" which could be filled with water. It was built for horses going to the Olympics/World Champs to start their fitness work at the tail end of winter (now everyone goes south) but was very fiddly to use so mostly ended up being a storage bin.
I think it depends how much exercise you want to do on one. I used to work at a research facility where horses would be walk, trot, canter and galloped on them the horses did great but for long term use I think it would be detrimental on soundness as its is really quite hard, softer than a road but much firmer than grass or an arena surface. Also when first using one we used 2 have 3/4 people to help after we would go down to 1 person in walk and trot but 2 people for canter and gallop, I would not like to leave a horse alone on one it could be sooooo dangerous