The app in the screenshot is populated with dummy data and I took it on my laptop so it using the desktop components, including the big think scrollbars, though on-device it uses the mobile components with more appropriate widgetry for things like the scrollbars.
It has a really nice column layout for browsing while making it easy to flick back and forth with your finger on the tablet. Once connected to the server, it shows the list of top-level categories on the left, such as "Apps", "Wallpapers" or "Books". You can then navigate through the sub-categories using a familiar columns system, which you can flick back and forth through on screen. Most top-level categories won't have many (often just 1) set of sub-categories, but for things like books supporting multiple levels was a requirement. The fluidity of it and some of the nice features like being able to tap on descriptions to get more information really only come to life when you start using it live.
It has integrated search, blends with the overall UI on the device, lets you manage your account in a screen that works just like the Settings app .. it's the kind of app we wanted as tablet users.
The Add-Ons App isn't limited to Spark, either. You can just as easily run it on any system with the KDE Platform installed. If you are a bit more ambitious, you could take the client library or write directly to the JSON API directly and create a completely different front end.
The API is still moving around a little, but once it is stable these "build an add-ons app" kit pieces as well as the Add-Ons App seen above will be made public under the GPL. This means that you can get in there and help make it even better or even make a completely different front end altogether, perhaps for a different type of device.
We will also be releasing the server-side code once we've gotten some experience with it in real-world usage. That means that organizations and individuals could self-host their own add-on infrastructure. (Textbook delivery for schools? In-house sales materials for companies?) We will also be offering custom store content configurations to those who would like that using our hosted installation. Since it isn't a package management system (though it can use them), it's quite agnostic in terms of how it is used and with what.