Kiva had the bad luck of coming along when Toei was in what I feel to be it's 'anime blending' cycle. They started adapting the feel with Kabuto, and were doing it trial-and-error until they got it just right in W. Otherwise I'm sure we'd have gotten a better series than we did. Not that it was bad, but....
I think Faiz and Kiva are chained (no pun intended) together by the fact that they both had alot of lost potential, and to me that stemmed from just having too much to talk about and not enough time to do it in. I'm guessing someone went through the list of things that needed to be tackled for the series, and they picked the ones they thought were most important. Unfortunately not all of those were as important as whoever it was thought. Or there were outside forces altering the story and overriding any such decisions.
As far as why people enjoy Faiz, I like it. Like many people here, I'm a particular fan of Takumi. Grumpy, unsociable, to go out on a limb, one might even say he's the Wolverine of the KR universe, albeit without the grizzled veteran side.
Anyway....one thing that occurs to me as I've read through this topic, (yes, he's on with another comic reference.) is that I feel like Faiz kinda equals Watchmen in a way.
Now I know that probably doesn't make a whole lot of sense, but the way I see it, Watchmen was, at it's core, a gritty realistic mirror being held up to the concept of Superheroes. What kind of people would REALLY do this? What would the world REALLY be like? What was the answer to those questions? Well, a falling-apart world, and a bunch of jerks and mentally questionable people as heroes.
Faiz I think shares in that sentiment. Everything from the Orphenoch Trio showing us how honest people would likely handle the situation of suddenly becoming monsters after dying, to, well....mostly everything. It's a depressing, realistic, anti-thesis to a franchise that's usually all about good guys defeating the bad ones. and despite reading watchmen many, many times and still not seeing where the rabid fandom gains their enjoyment of it and where it gets it's acclaim, I see where Faiz gets the credit it does, and I feel like in Faiz's case, it's deserved despite the flaws already discussed.
Now, obviously...*gets out the fire extinguisher*...this is all just my take on the situation. It's how I feel about the series. Guess the bottom line is I do enjoy it, mostly because it's a different take on the franchise and there are elements that I enjoy. Admittedly they did have a bit too much to talk about and it wasn't all covered, but I liked it.