It outsold all expectations and was successful enough that it brought several traditional PC makes (Asus and Lenovo) into the market.
It's obviously sold less than established consoles, but it's done well for itself. Also even though it's a small percentage of steam users, it seems to represent some of the more active players/spenders on steam. Thanks to that, steam deck players seem to be an actually significant percentage of steam sales for many games despite the lower number of steam deck players.
It seemed heavy at first, but I don't really notice the weight at all anymore. I find it much more comfortable to play than a lighter handheld like the switch, ergonomics are more important than weight imo.