Pros: horses seem to like it, very mild bit, good for breaking and training
I train horses for hunter/jumper, Western pleasure, trail, and driving. I like to use the mildest bit I can, and this one is about as mild as you can get, save for a rubber-coated bar. It also gives the horse something to play with, with the copper rollers.
The full cheek is great for keeping the bit in the horse's mouth--I have seen D-rings and loose rings pulled through mouths. That won't happen with a full cheek.
This bit puts the pressure from the reins on the sides of the horse's mouth, so you have to use direct reining (one rein in each hand, pulling back or to the side while riding) while using this. It is not a curb action bit, which puts pressure on the horse's poll (top of head), chin (from the curb chain) and roof of the mouth (depending on the curb type).
Snaffle (jointed) bits are the mildest and good for young or green horses. They are also good for lesson horses, as it is harder to hurt a horse with a snaffle than a curb, so there is less chance of a beginner ruining the mouth of a good horse. There is no leverage action, so whatever pressure you use to direct the horse is the same exact pressure the horse feels. (Curb bits act as levers, which amplify your signals, and can hurt a horse if an untrained rider starts jerking on the bit).
I've had good luck with Korsteel bits--no rusting, no chipping, and I have yet to have one pinch the corners of a horse's mouth, even though people have told me that full cheek bits are "bad about that". All Korsteel bits are high quality, easy to clean (I use a toothbrush), and stay shiny through the years. They are easy to find at tack stores and in catalogs (such as State Line Tack). They are moderately priced (usually 20-30 dollars depending on the type of bit).
I haven't had any horses put their tongues over this bit, or chomp down on it and hold, so they must like it. I have had some horses get antsy with other bits, like Kimberwickes, but I haven't had any attitude problems with a copper roller Korsteel.