Do a forum search for "raspberry leaves" or "Mare Magic" and you'll get TONS of hits.
Personally, my experience with Mare Magic has been pretty great. Lacey can be a very high strung mare and that quality certainly does not disappear when she's in heat.
I find that it doesn't so much change her behavior as it changes her reaction to things...if that makes sense.
Like, one of her favorite tricks, when in heat, used to be spooking violently at "something" (read: nothing) behind her and bolting forwards with no thought to what was in front of her (she tends to be much spookier when in heat). Once I got her on MM, I found that she'd still spook violently at nothing but it seemed like she'd would take a split second to determine if she could bolt forward. If I was in the way, she'd simmer her jets where she used to run me over on a semi-regular basis because she'd just blindly bolt.
It seems to me like being on MM consistently for about a year has basically taught her new, better behavior patterns for dealing with her feelings/urges. Like the spooking thing, or the rearing thing, or the just generally being a brat thing, she considers those things when it's her time of the month but she seems to use more sense in deciding whether those behaviors are appropriate.
Now, I could take her off it for a week or so without any ill-effects where a week off used to be an invitation for craziness. However, if I leave her off of it for a while, the bad behaviors do start creeping back. Really though, the MM has made her so much easier to work with that I have no issue paying for MM.
Of course it doesn't work as well for all horses, but this was my experience.
Buy crushed raspberry leaves on Amazon. It's exactly the same as MM (MM is just crushed raspberry leaves) but MUCH cheaper. Feed about 1/4 a cup for the first week, then 1/8C is probably all you need. Personally, I have one of those 2oz scoops that come with supplements that I use with my raspberry leaves (I bought legit MM once and kept the scoop to use with the leaves).
1lb will last you about 5-6 months, for the about same price as 1 month of MM.
I'm linking you to one on Amazon that's $12 (free shipping if you have Prime) for a pound: