Since Tara's feed is already fortified, I only just started giving her a multi-vitamin. I feed her Legend's Show & Pleasure textured feed, but since I don't feed her the 'recommended' amount, I give her half the suggested dose of Vita Plus (I think it's made by Farnam).
Honestly, I have no idea if it's doing it's job. I didn't see anything wrong with her before I started giving it to her, but I was paranoid that was she lacking in some major essential vitamin. She's still her happy, healthy self, so if it made a difference, it wasn't a big one. Since she seemed fine before, I didn't want to spend a lot. I think I spent 15 bucks on a bucket and since I only feed half what they say to feed, it's lasting forever!
I have heard good things about Grand Meadows products, specifically Grand Complete. They do make Grand Vite though, which is I'm assuming doesn't contain all the extra ingredients for coat, hooves, etc.