I've mostly had good experience with Fasttech. I've ordered a few atomizers through them (mostly Kayfun clones), and a few mods (such as their 1:1 Stingray X clone, which is an amazing value). They are legit.
Key things to understand:
1) Don't be in hurry, especially if it is a new item that is showing a 5 or 7 business day stocking delay before shipping. That can easily stretch to weeks, especially if it is a hot in-demand item. It's a crap shoot, had one item take almost 2 months to receive, yet another get to me in 2 weeks. Generally, items that are in stock and ready to ship arrive in 2-3 weeks.
2) Read the Reviews and the Discussions for the item. The Discussions, particularly, are both entertaining (some of the posts are downright funny, especially if somebody brings up brass concerns) and amazingly nit-picky/insightful. It will also give you an idea if a particular version of something has issues, as these guys WILL find the most minute concern from the pictures alone.
3) Many products have multiple "versions", usually just labeled A, B, etc. These are different manufacturers, but good luck knowing who makes what, as that seems to be a closely guarded secret. If you're looking for a particular manufacturer for a clone (e.g. Infinite, HCigar, Tobeco, etc), the Discussions can be helpful but not always accurate as they are reading tea leaves to figure it out. If the manufacturer is important to you, buy from a reputable online vendor instead.
4) I usually try to stick with items labeled "1:1" clone. They seem to be better quality, almost universally.
5) I find that the China Post ePacket via USPS shipping to be the fastest of the free shipping options. Avoid SingPost like the plague; had something stuck in there for weeks once. Regardless of your choice, tracking data will suck until it hits US shores.
Good luck!