Well I don't want to scare you in to not trying it out, but I do want to explain to you why I personally did not have a good experience. I can't speak for everyone because each medication affects individuals differently.
I became extremely anxious and depressed. I began having panic attacks and had a general feeling of losing myself/going crazy. I had a hard time concentrating on work or school, which gave me even worse anxiety. My communication/speech became impaired. I could not find words at times, or when I tried to say something I studdered...often sounded dyslexic. On top of all of these negative side effects, I never got any relief from topamax. I never noticed any change in the frequency or severity of my migraines.
But I think when you start trying out different preventatives you can't discriminate. You have no idea how your 17 year old will be effected. I am not sure if you have found any positive testimonials on topamax - but they have to be out there since it is used so heavily.
I think one mistake I have made when getting new treatments from doctors is not being thorough enough in the questions I asked beforehand. I'm 21 now but I still bring my mother to doctor's appointments. When my doc. put me on topamax I didn't ask about any of the negative side effects - I just started taking it. Just be observant and critical. Other than that you just have to experiment to find out what will work the best