i've suffered migraines for years, and here is what I've been told:
make sure it is done by someone who knows what they are doing, it is a science not an art, and the placement is very different than for cosmetic reasons, that not to say you can't get a little cosmetic assistance while you are there but using botox for migraines is different than for cosmetic reasons, (my Nurse friend who does cosmetic botox gave me that advice) that being said I think there is a pretty good success rate once the right nerves are found, and botoxed, so you may have to try more than once for complete relief. I did hear of some good results out of UT Southwestern with botox in the neck/back of the head area, but I don't remember the details. as far as negative reactions, it think the risk is about the same as regular cosmetic use of botox, so in my opinion it would be worth a try, I say go for it, just be sure that the person injecting you has a lot of experience working with migraine pain not cosmetic botox.