Well, I am a frequent buyer of clones, and most of the time, there's no problem. There ARE a few bad clones out there, and I have bought a few. In the last year, the clones have gotten better, and some are even better than the original. When you know you're buying a clone, look at reviews of that clone. See what others think.
One last thing to mention, is if you buy a clone, be prepared to thoroughly wash it. Some clone companies don't clean their items very well, and a distinct flavor of machine oil will make it unvapeable. I've made it a habit to clean ALL atomizers when I get it... sometimes to the point of boiling it two times when the flavor is really bad.
Here are some of my experiences with clones... some good, some bad.
First clone was a Nimbus RDA. The post holes were "press fitted" in. After a month or so, the posts weakened to the point where I couldn't use it. Also, the posts had allen wrenches... When I lost the wrench, I couldn't find a replacement... so, that RDA turned into a spare.
Another was a Kayfun lite clone. The manufacturers didn't bore out the chimney post long enough, so it never made a really good seal. it caused the atomizer to leak quite a bit. A simple O-ring on the bottom to raise it up fixed the problem.
The Squape-R. I so wanted this one to work. It didn't have the logo emblazened on the side, and had glass instead of plastic. Unfortunately, something is wrong with it and I cannot figure out why, but it leaks like crazy. After about 30 builds, with either totally dry hits, or tank emptied down the side of my mod in the morning... I gave up on it.
Derringer... I prefer phillips head screws since they're easier to work with. The original derringer used allen wrenches, so I view this as a bonus for me. Also, I got four different options for a top cap..... Stainless steel, copper, brass, and white delrin... all for $14... it's my favorite dripper, and MUCH better than the original.
Goblin mini RTA. Even though the goblin mini is fairly cheap ($25) I bought a clone for about $9.... almost a third of the original. It's glass, and I've had zero problems with it. I've heard others having problems with the drip tip falling off of the original.. mine is a perfect fit.. I can pickup the mod RTA by the drip tip, and wiggle it quite fiercely and it still holds. I bought it as a small "to go" tank to fit on top of the cloupor mini for a small vape, but it's so good, it's become my main RTA.
So, the answer to a clone, is some clones are better, some worse... just do research first. If you want to be %100 sure, then buy the original.