I've been using D8's for a few years and I think they're outstanding.
That's not to say they're the perfect choice for every kind of game, though, as no joystick is.
Initially I bought them to replace a pair of Happ Competitions on a 2-player 8-way setup, mostly to improve the Robotron experience. That panel is 5/8" MDF and the joysticks are bottom-mounted (no routered pocket). I ordered the round restrictor to keep the throw a bit more snappy on the thick panel.
They worked OK in this setup, but eventually I decided to move the Competitions back to that panel and build a separate Robotron panel using the D8's with closer joystick spacing (10" if I recall correctly.) I ditched the round restrictor and top-mounted the D8's to 3/4" wood using ad hoc mounting plates I fashioned out of electrical junction box covers. The top-mount with no restrictor provided a very comparable feel to the bottom-mount with restrictor.
Shortly after doing that, I sat down and monkeyed around a bit with the leaf switch adjustment on the D8's and this is when they REALLY started to shine. After 3-4 iterations of minor tweaks to the switches, it was joyous. I did a quickie adjustment when the sticks were new but never revisited it to tweak it at all. I don't know why… I was missing out big time. Bottom line: if you buy D8's, SPEND A HALF HOUR TWEAKING THEM. It's not difficult, and it makes them really sing.
Now, with dialed-in switch adjustment and nice comfortable spacing, twin stick games (Robotron, Smash TV, Total Carnage, Black Widow, etc.) are a blast, and very faithful (all things considered) to the real thing.
The D8's are also outstanding (IMO) for shoot-em-ups. Which makes sense, because a lot of the traits that are desirable in a Robotron stick are desirable in a shooter stick (precise movements, twitchy response to dodge stuff, etc.)
They're also totally fine with side-scrolling beat-em-ups, although I don't feel that they provide any real advantage in games like that.
I wish I could provide an educated comment on their suitability for one-on-one fighting games, but I really can't. I just don't ever play those games and don't really care. I will say though that I *suspect* the D8's - at least, the way I have mine set up - would not be a top choice for fighting games. I think something with a bit of tactile feedback based on joystick position (such as my Happ Comps) would be better, but again, I don't really care either way.
Another thing is, in my opinion the D8's aren't a very good choice of 8-way if you plan to "fake it" using an 8-way joystick to play 4-way games. I can much more easily manage this with the Comps. But a joystick's efficacy at faking something is probably not a good criterion for judging it.