I've still got a pre-order in place, but I'm really on the fence about whether I'll bother to keep it. I have no doubts that streaming will replace traditional consoles at some point, but at this point I don't think Stadia will be the platform to do this. Assuming that xCloud works as well, for instance, it's going to offer a similar experience but with the added bonus of Game Pass and a bunch of high profile exclusives.
What's tempting me to keep my pre-order is the chance of playing the next year's worth of games with better graphics and performance than the current gen consoles can provide. If there's no point using it after that then I'll still have a Chromecast Ultra for the money, along with a (hopefully) decent pad I can use with my PC.