I don't think this is a devtools* replacement or a significant enough "game changer", this is just a different way to interact with CSS. Devtools reveals both the HTML & CSS (as well as the cascading rules that comprise what I'm seeing). Devtools let's me introspect & debug why what I'm seeing as it's happening with CSS in conjunction with HTML & possible HTML changes caused by JavaScript events or pointer-events, for example. I like that this tool could make looking at the parsed & rendered code in a browser window easy to "see" & maybe copy. But for a small part of what devtools does this really is expensive. More expensive than most other apps I use. More expensive than GPT+ plus. More than the cost of hosting my site on DigitalOcean or AWS.
If this were a truly must have app & I could get past the price, I'd enjoy it with the concern that most features could be incorporated into devtools directly, by browser makers or by other extensions that expand what devtools can do, although many features of this tool already can be achieved. In fact, I think that's the real missing feature in this service: it could be an web extension for augmenting CSS work devtools.
Lastly, what if the price started at $15/month and for every 250,000 (maybe less, maybe more) new users everyone's price was dropped by a $0.50/month but could never drop below $3/month. By the time the price hits $5/month there are at least 5 million subscribers ... (with no annual plans & no life time bananas plan) that seems pretty good.
* devtools is pretty awesome & pretty consistent in every browser.
(Oh, and I'm super cheap, too.)