MTG works absolute wonders. You see a visible difference in just a couple of days! It is infamous for sun bleaching and sunburn but I haven't had any of those problems with my two horses (a bay and a red dun) because I use Santa Fe detangler along with it which is a great detangler and conditioner that also has sunscreen in it. My bay's black tail is as black as possible and I use MTG weekly. Neither horse has been sunburnt or anything like that.
It is also very greasy. The smell kind of sticks on your hands. I use soft scrub with lemon to get the smell off and it seems to help. Because of how greasy it is, it's also good to wash out the hair a little bit more often than you normally would to keep it from getting too icky.
It really does work wonders, though. My horse's tail was pretty damaged from rubbing because he was wormy and neglected at his previous home, and all of those short pieces are really filling out and it's looking just beautiful. I would definitely give it a try. You can use it once a week as prescribed on the bottle, or I've also heard people say they use it every other week or once a month and still see noticable results.