To give a little background: I LOVE Tokusatsu media, especially ones before Kamen Rider Saber. I think every single one of them are great, especially the Kamen Rider ones (Zero-One being my favorite), after I've heard that an actual Kamen Rider anime was going to be released (which is this one), I was pumped, and excited for it, in fact, this is the first anime that got me into anime in general

"Underrated" and "Under-watched" are terms that's being tossed on every single thing Tokusatsu-related, and this loosely applies to this anime too, as I was also a big advocate for Kamen Rider W

(there are some spoilers)

To start, let's look at the animation:

Every single part of this anime looks decent at worst, at best? very accurate to the source material (that being, Kamen Rider W), the suit designs just look AMAZING, and clean.

It pays homage to the source material A LOT, the Rider Kicks, the forms, the sounds, weapons, and everything

Spoiler, click to view

especially Kamen Rider Double's Final Form, CycloneJokerXtreme.

the new Dopants are also very creepy, intimidating to look at too.

Now the characters are what that surprised me:

The old (Shotaro, Phillip, Ryu, and Akiko) have kept their old personalities, and the show makes them feel very fresh to see, only this time is funnier, Shotaro and Phillip have pretty nice chemistry that doesn't feel too different from its predecessor

The new (Yukiji, and Tokime) are also quite cool, Tokime in particular is a pocket of fanservice that actually contributes to the plot quite heavily, while Yukiji has a good villain introduction.

One small complaint I have, is that i wish they used the old cast in Kamen Rider W for the voice acting (for Shotaro, Phillip, and Akiko that is), nevertheless, the new ones are good.

For the story:

It expands on the current Kamen Rider W lore, while also not retconning anything, every new plot point being added serves a purpose, and i love that they used the old assets from Kamen Rider W (Gaia Memories as an example), to make them feel fresh

Shadow Fuuto is also brilliantly told, dark, and bleak, with a lot of depth inside

This show in general follows the same kind of "path" as a typical Tokusatsu, it's a "Monster-of-the-week" approach, and it kept the theme the original had.

Overall, If you liked Kamen Rider W, you will certainly like this.
Although I liked the story, it's highly recommended to see Kamen Rider W first (and maybe even the movies too), THEN watch this, because this anime was made as a sequel, not a standalone project, so you might not understand everything.