This review is written as a long term viewer of the franchise and has seen versions other than the American. So I will focus on the Japanese version like in my prior reviews. I won't talk about characters and story because I already talked intensively in the Dragon Ball Z review.

So Dragon Ball Kai, or Dragon Ball Z Kai in the US, is nothing more than a cheap cashgrab by Toei to rerelease Dragon Ball Z under a different name.

The original appeal would be that Kai is a "faithful" adaptation to the manga, but cherry picks what scenes are really important and usually not very wise ones.

On paper, Kai suppose to make the dragged out fights and episodes better paced, but by removing key filler from Z that made it so engaging, even the best amount of scene removals don't help because the manga pacing of Dragon Ball was already horrible. The anime filler such as in the Saiyan Saga had an intensive training arc where characters not only train to get stronger but also prepare themselves mentally.
Without these scenes Piccolo's death feels hallow since there is hardly any interaction with him and Gohan.
The snake way didn't feel as long as it was portrayed.
The travel to Namek feels like driving to the to the the convenient store in your own town instead of going through space aka. uncharted territory.
And some of the best episodes like the Drivers License episode were completely removed. The best episode of all of the Cell Saga. Not to mention the Otherworld Tournament.

Modern censorship is also a reason not to bother with Kai. Unless you can't look at cartoon blood or think in a show fighting for live or death shouldn't have blood, you can consider watching it.

There is also the false advertisement with the opening, making it look like Z would be completely reanimated. But when you watch the show, to the naked eye you can hardly find any difference between Kai and Z in terms of animation and art quality. That is not to say they didn't improve. It's just not noticeable unless you put them side by side. It looks nothing like in the opening.

Another reason I heard that made Kai amazing is the lack of the Boo Saga. Sadly since Kai got a sequel and continued, that hold no water anymore. Thus the awful Boo Saga being part of Kai drags it down since the Boo Saga is the worst paced and dragged out fight in the manga.

I honestly don't feel anything from Kai. It underperforms in every category to Z. But its still Good Dragon Ball, and even it being a soulless cashgrab, I can't give it lower than a 5. It's not awful but not something a modern anime fan can get in. If you are a fan of a slow show that gets better, you are better off watching DBZ.