Meh. I've been giving them to my mare for about 3 months now (Smartcalm Ultra). Not sure if they actually do anything. She was dead calm over the winter so I was thinking "score they work!" but come a couple weeks ago, she has gotten a lot spookier (trainer I think its due to hormones) and harder to ride. But, she is still a lot more rideable now than she was last year at this same time. So...helping or not? Couldn't really say.
They're fairly inexpensive for the most part, so might be worth a shot.
I have seen the supplements make a difference on some horses and do absolutely nothing with others. I rode one mare who still had PLENTY of go while she was on SmartCalm, but it did help manage her spookiness a bit. As for my mom's gelding - it did nothing for him under saddle but made him seem stoned on the ground. It was miserable and we took him off of it almost immediately.