It looks really interesting, but I do have concerns:
1) Price - People keep saying that it is competitively priced and that Valve are almost certainly going to be making a loss on it. It is competitively priced for a gaming PC, but not compared to the only other really viable handheld gaming device; the Switch. If you're going to install AAA titles on it, you're probably going to want the faster storage option of the 256GB, rather than the 64GB/Micro SD option.
If you're not going to be putting AAA titles on it, and stick to indie titles, you may as well go with the cheaper/lighter Switch.
2) If you have A.N.Other games console, the AAA games are likely to be similar prices and run better than the Steam Deck equivalent, albeit that another games console isn't portable. Bontakun3000 above mentioned that a lot a switch games are cheaper on Steam. I'm not sure, but there are often Nintendo eshop sales where indie titles are very cheap, especially if you use alternative regions (Russia/Poland etc).
3) The weight, shape and size - Seems to be almost twice as heavy as the Switch, longer, bulkier and the button placement looks to be less than ergonomic.
Having said all that, I do love a gadget and games, and am massively tempted to get one.