For me, KS has not really paid off and I avoid it at this point. I backed a game in 2014, that has yet to arrive - over a year late - and I'm not even interested in it at this point given that my tastes have changed in that time. I backed Epic and was sorely disappointed. I backed Tiny Epic Kingdoms, and found it "meh." It's not a bad game, but just meh. I backed I Say, Holmes and also found it lacking. I purchased Viceroy at retail, and was really surprised at how mediocre it was. Overall, I haven't seen anything on KS that made me feel like I really need to engage with it over regular releases by established companies.
At the very least, I don't feel an urge to get a KS game until it is fully released and vetted by the general public, which is something I am doing with The Pursuit of Happiness for example.
I would make it clear, however, that I differentiate between KS-alpha and KS-beta games. KS-beta are games on KS that would have been published anyway because they are being done by established designers and companies like Eric Lang, Jamie Stegmaier, CMoN, etc. but their model is to use KS. I personally don't think it's fair to use these games as benchmarks for how successful KS is as a means for producing good games. If Eric Lang wanted to get Blood Rage or The Others: 7 Sins produced he could have without needing KS. And he has access to developers, publishers, and others in the field that unknown designers don't have.
KS-alpha games are games designed by unknowns who use KS as it was originally intended; a forum for people who did not have access to publishers but had a great idea/product and KS could give them the leverage to get the game produced and even open doors for them to get into the field. Personally, I think KS-alpha games are becoming more derivative, simply being another version of something already on the market. And many of them still have issues with the final product (i.e. a glaring mechanistic "kink" that an unknown designer just couldn't work out without having developers to work with). They can certainly be playable, but I don't think they have staying power or ultimately will satisfy the intended niche.