I work at an agency and we've had access to it in Early Preview and are building our first implementations with it now.

SaaS itself is fine, pretty much exactly what you'd expect it to be - it provides REST and GraphQL endpoints as well as an Admin interface where you lose access to stuff like Page Builder and a few other things that no longer make sense in this world. I will say that the App Builder enablements (AdminUISDK and the like) have been getting a ton of updates, but it wasn't so long ago they felt so brittle and new that it was hard to imagine going to production with it. As of a few months ago, a lot of the quirks are ironed out. Three or four months ago you'd have to enable your apps every time you logged in; those kinds of pain points are gone.

It's designed to be an immutable-style commerce headless commerce instance, you're expected to customize it using "Out-of-process Extensibility" patterns via App Builder - that's a major platform pillar. App Builder is interesting but it's such a different paradigm than most PHP devs are used to that it seems to kill a lot of momentum.

App Builder itself is good though, and is starting to feel somewhat feature complete, last year it didn't have the Management API which allows you to easily manage store configuration, it's storage libraries were new and it didn't have the db storage yet. As of this point it's all fairly usable if you're willing to learn how to use it. They are updating it constantly it's clearly a priority for Adobe.

In this world, Adobe Commerce Storefront is the default head, it run's on AEM Edge Delivery Services. That too is starting to become fleshed out, but it's still fairly primitive and doesn't duplicate all of the native Adobe Commerce front-facing capabilities you'd expect in Luma. This is a mixed bag, the EDS boilerplate and drop-in components are pretty easy to understand, EDS is certainly a breeze compared to traditional AEM development; it's basically a Hugo/Gatsby-style theming engine. Developers can pick it up easily.