I actually quite like the concept. Sure, it's basically just an X86 laptop (or tablet) in another form factor, but that's not necessarily a bad thing.
I don't think laptop keyboards and touch screens are ideally suited for gaming, so this definitely fills a niche. I also really like the fact that it's been built in a modular way, with user replaceable parts. So, if for example, the d-pad failed, you wouldn't be screwed.
I will agree that the 512GB model is overpriced. However, the base model is decent value, and there's nothing to stop you from adding your own SSD separately.
Valve's support is a non-issue for me, because if I bought one of these devices, I would mostly be running regular Windows or Linux on it.