I'm on Topamax, Atenolol, and Citalopram...all for migraine prevention. We added the Topamax to the regimen (100/day) about 6 months ago. The first 6 weeks were the worst for side effects, especially the metallic taste, tingling hands, 'stupid' feeling, and fatigue. However, I did eventually adjust, and the migraines did decrease in frequency and intensity. They were not completely eliminated, but when I still get them, I find that I can abort them with the 'rescue' drugs easier, and they don't last as long.
If you like soda's and are upset by the 'flat' and metallic taste caused by Topamax, there are some sodas that don't taste quite as bad. Coke Zero and Diet Pepsi Max seem to be a little more carbonated and have a 'zingier' taste. After about 3 months, some of the carbonation 'feel' came back and I could drink things like beer again, without it tasting like I was sucking on a rusty nail.
I use to get the 'stupid' feeling to for about the first 2 hours after I took my morning dose. I couldn't remember words, and would forget what I was talking about in the middle of my sentence. I started taking my meds at 4 am, so that I had a chance to sleep the 'stupid' off by the time I got up at 6 and left for work by 7am. Then I'd take my evening dose after dinner, because my kids don't care if I'm an idiot...just as long as I'm not cranky :-)
When I have a migraine, I usually want to drive a pick-axe into my head, so all-in-all, these side effects aren't too bad in comparison, and make me a much more pleasant person to live with.