My dd-3 has been on Topamax for a year. This was her first anti-seizure med, so I can't comment on the transition. My dh is on Tegretol though, so I can see some comparison in the differences and I can sympathize with how scary switching seizure meds is since we've gone through it with him.
Becca is on 60mg a day (she is 26 pounds) but built up to that over a period of 3 months...we started at 15mg and went up by additional 15mg until we reached the full dosage. It's less of a shock to their system that way and good for seeing side effects progressively.
The pros we've seen:
--no seizures in 10 months

(the obvious biggie)
--doesn't make her tired (which 1600 mg of Tegretol definitely does to dh)
--she is happy and well adjusted on it
The concerns...
--NO SWEATING...this is very common on Topamax and very hard with those that can't communicate their needs. We spent the entire summer indoors because even 70s in the shade causes her to overheat...she also overheats when she gets upset. You have to pay very close attention to any signs of heat issues.
--Learning/Speech delays...not sure how much of this is directly related to the Topamax but I know that it does cause problems recalling words/information. People describe it as being like when a word is on the tip of your tongue but you just can't think of it. We don't know how much is meds or if it was her last really bad seizure or something else but she hasn't been able to retain any new information since last year. She was classified as preschool disabled and is talking more now but still not making progress in the other retention issues.
--Other possible side effect/issues to watch for...Topamax can cause kidney stones and require that you stay extra hydrated (doubly true with the no sweating!)...we haven't experienced this but it is something to keep in mind.
--As JenJen mentioned...it can change taste and cause weight loss because of a reduced interest in eating. I was told that as long as Becca didn't lose more than 10% of her body weight not to be concerned.
Those are the major side effects I can think of off-hand or that we've experienced. Even with them, we love that Topamax has given us a little security against Becca having seizures. All of the meds have different side effects that could make or break the deal depending on how they affect your own child...it's such a hard process--I'm sorry you are going through such a rough time and hope that the Topamax helps control your son's seizures
