I've had a bunch of teleconverters at this point. As long as you're pairing it with a lens that works well with it, it's easily the best I've owned yet. Using a TC ends up being an equation, a balancing of the benefits of reach vs the drawbacks of slower aperture and IQ degradation. The best TC's are the ones that have the least impact on the image quality. That's all pretty common sense, but it needs to be kept in mind when evaluating. Has worked fine with all of my HSM lenses, but Sigma has a compat list on their website that tells you what you can get with regards to AF with various lenses. It's reliably worked with my Sigma lenses (120-400, 300mm macro, 105mm macro, etc.)The Sigma 1.4x has consistently given me the best balance of extending reach and minimized degradation. If you are using with other brands, there's no way to know if its phsyically compatible or if you'll get AF, or how much impact it will have on IQ unless you try it out to find out what impact on AF. It's always a crap shoot. There are a few lenses this doesn't work on, but its worked on all that I really wanted it to. I have this for the Sony\Minolta A-Mount, but more often use it with A-Mount lenses adapted to either the Sony A7ii or Panny GX85.

It has a negative impact on image quality. This really comes down to the lens being used and how well it interacts with the TC. Impact from 1.4x is less bad than with a 2x TC though. TC's magnify existing lens flaws, so if you have a lens that has a lot of SA and reduced contrast wide open, then it's going to be much more noticeable on the TC. You also lose about half a stop, so slower lenses can lose AF capabilities or might hunt in low light. A lens that you think should be great might be terrible on a specific TC (you don't get the reverse of that, a bad lens won't be fixed with a TC) but might be fine on a different brand or model TC. You have to test to see how the performance is. Some lenses don't report EXIF or correct FL\Aperture with the TC attached. Some do it fine, another thing that just needs to be tested to know, with both the lens and the camera body.