I use them every spring during shedding... and pretty much yearround when it comes to dirt. I'll swear by them like I do with MTG now... They work and work good... and are the best things in the world to held shed a horse out with. They get impossible dirt off very easily too.
I love these things! I use them whenever I have them but I use them a lot during the shedding season as they help remove a lot more hair. Combined with my shedding blade I get a lot of those deep down loose hairs, plus they get rid of a lot of dirt and leave a nice shine on the coat. They are little pieces of gold.
I often buy one in the spring when the horses are shedding. I use a metal shedding blade, a rubber curry, and the slick and easy because they all work for different amounts of shedding! When they're really shedding you get the most hair off with the blade, but once the majority of the big clumps are removed the rubber curry gets out quite a bit. Once you're not pulling as much hair out with that I move on to the slick n easy which pulls out a lot of hair that the curry doesn't. If you just use the slick and easy first it gets 'clogged' from the massive amounts of hair. It does wear down pretty quickly though and will break if you drop it on concrete.