I have been ordering from them for quite a while with only one problem, and they handled the problem professionally (mini protank clone came defective, they shipped a replacement free). Just keep a few things in mind.
1) they are a reseller not a manufacturer. They state what they see. If it's authentic, and their vendor says it's authentic, chances are, it WILL be authentic. I have yet to see them state false information.
2) Unless you live in China, it will have to go through TWO customs offices... one in China, and the other from your country. That will delay shipment.
3) they usually ship on time...sometimes 1-2 days later. if it says "ships in five days", don't expect it to start it's mail journey until five days are up.
4) most shipping carriers have labeled Lithium Batteries as a hazardous material. Because of that, they refuse to ship them without paying an extremely high ammount. Fasttech has found other carriers that still ship lithium batteries, but their tracking isn't as good as SingPost or USPS.
5) expect longer delivery times... you're getting a great deal, but it will require more patience.
6) read customer reviews... they're your best source for finding the "low down" on the item.
Just my two cents worth.