Mpt lucked out in being practically immune to the side effects, but if you read the user experiences and this thread, the pattern is quite clear: everybody who has been on at least 700mg has reported a decrease in their tinnitus, some have reported improvement at lower dosages.
The only people to have had no positive effect that I recall are Zimichael and Hengist. Zimichael had supply problems and scheduling conflicts that made him have to stop at 600mg, and Hengist, frankly, is so unrelentingly negative in his posts that I don't know if he'd admit to any cause for optimism. Lep withdrew due to side effects but later tried again and had some success, and Viking had to stop early but had experienced some promising results.
On the question of whether the effect can become permanent, it's just too early to say. Christian78 has talked about taking a break. If and when he does he can comment on whether or not his levels stay where they are or not. Johno made a comment to the effect that he kept his improved state during a break.
@locoyeti, I apologize if I missed a post, but did you ever say if what you're taking is branded Potiga or Trobalt?
I'm doing research on importing and so far I don't see any reason an American with a prescription would have reason to be concerned about importing from Canada, the UK, or an EU country. The FDA says basically that while it is technically illegal, it's not worth time and money to stop anyone importing a substance that is for their own use and approved in the US. Even for unapproved substances they have guidelines that take into consideration compassionate reasons for turning a blind eye. They repeatedly say that their top concern is public safety, and in this case I actually believe it!