I tried to get a tankbag for my 640 several times and when I finally got to the KTM importer in my country and they said they had one, it turned out it was for a 950. (After I drove there for 2,5 hours)
I ended up getting a Touratech tankbag and I think it does the job well.
I think the KTM importer gave me a good enough reason to go for an aftermarket bag, though I'm not going to vote cuz I can't really compare it to other bags on the market.
Welcome to Poland...
KTM 640 - The wheelie monster
Husqvarna TE 250 - The little beast
Husqvarna 333R - The ...umm..... how do I put it....