I've been trying to wrack my brain to answer this question. For some reason, Kai never clicked with me. The dialog itself is a big improvement over Z's in terms of the cheesiness and accuracy, and the voices are more seasoned and one can make a strong argument that the replacement cast is better. However, it just doesn't do it for me. Here's the issue for me: if I want definitive Dragon Ball/Z, that's the Japanese version. Nobody's touching the Japanese version. That and the manga, to be quite frank. As a matter of fact, over the last couple of years, the manga has pretty much taken over the way that I enjoy Dragon Ball as it's a lot more convenient (and accurate, if you care about that) than watching it. So if I'm looking for that super faithful Dragon Ball Z experience, my go-to is the Japanese version. And yet, I love the dub. Why? How can that be? I think I got the answer. I love the cheesiness, I love the inaccuracies, I love that it feels like a totally different series than the Japanese version. From the music to the line delivery, to the voices themselves -- I mean, damn, it comes off as this ultra-cheesy Saturday morning cartoon rather than Dragon Ball. And I freakin' love it. I'm not saying it's done "correctly" or that it even lines up with the Japanese version, but I just love the unintentional parody that Funimation created with its attempt at Z. I kind of look at it like they got to the series before Team Four Star did except where TFS parodies the dub (mostly), the Funimation dub parodies the Japanese version. Hell, I don't even like watching the redub that they did for the second and third "seasons." I love hearing that horrendous Vegeta performance that Sabat did (that's why I watch the singles for those episodes). It's the power of nostalgia for me and I admit it.
So, if you grew up with the Funimation Z dub and actually loved it, why would Kai replace it for you? It's no mystery in today's world, that Funimation's original Z series is not done faithfully to the Japanese version. If you love it anyway, how can you really switch over to Kai as your standard when it's so totally different? Now, if you're a fan who's been annoyed at the fact that you didn't get a faithful representation of an English-speaking dub, then yeah, Kai's for you (if you haven't been pushed away to watching the Japanese version exclusively out of habit). And of course, if your first introduction to the series was through Kai, I mean, it's pretty certain that it's going to be your go-to.
Personally for me, if I want the faithful telling of the "Z" story, the Japanese version is pretty accessible. Hell, I have five releases and two streaming subscriptions that can fulfill that for me. But if I'm looking for a dub, then I'll take my hilarious inexperienced Funimation one. Well... I'm kind of more into the Ocean/Westwood dub of it than Funimation's, but that's just more nostalgia (despite not growing up with the Westwood dub, I sure did love the Ocean dub of the first two arcs) coming up to take me away on a wonderful ride.