That’s fast and convenient, but also a hindrance. Specifically, my iPhone, iPad and Mac are now cluttered with space-hogging one-offs I shot for the sake of a tweet or a Facebook update. 1 What’s worse is that you can’t delete such throw-away photos from your Photo Stream with an iDevice. Instead, you’ve got to visit icloud.com and click “Reset Photo Stream,” which nukes the lot, good and bad. That’s why I’ve started using Camera+ again for tweeting pictures.
As Dave Caolo mentions above, regardless what you shoot, it will end up in your Photo Stream. Apart from the random twitpics, also the screenshots you may capture to show a bug, feature or just your favorite homescreen layout.
But there's a bigger hindrance. Not so much with the Photo Stream itself as with iPhoto.
use case:
my wife, my two kids and myself all have iDevices and we all have our own iCloud account (MobileMe Family but that's defunct now).
We have one computer that has our iTunes collection of music, movies, TV shows and apps. And I would like to use that same computer to collect all Photo Stream content.
However, iPhoto only connects to one single Photo Stream. You just "enable" Photo Stream, and it will use the one that is currently active on your computer. There is (afaik) no option to import other Photo Streams.
This makes the whole feature a lot less interesting and useful.