Topamax is one of the most effective preventatives on the market. As Sarah indicated, many people come off of it because of the side effects even if it's preventing headaches.
At higher doses it's been known to cause cognitive deficiency (fogginess), nausea and loss of appetite and change in the taste of some foods and beverages (leading to weight loss), parathesia (tingling in the extremities), depression, lightheadedness, etc. In most patients these symptoms subside over time, but, for many of us it's too much to handle. If you suffer from painful headaches, however, topamax is at least worth a shot.
DX: NDPH, Recovered CRPS
RX: Lamictal, Abilify, Provigil, Clonazepam, Ambien CR, Emsam, Namenda
PRN: Haloperidol, Zyprexa, Lodine, Zofran, Oxycodone, Skelaxin, Diluadad