The size and weight are great, the iPad feels bulky after using the Fire. It's great for reading while holding in one hand, something I don't do much with the iPad because of it's size and weight.
I'm not sure why hardware volume buttons would help much, the software controls are always one tap away. I'm not a fan of the only physical button (power)--it's small and on the bottom, but I'm probably just used to the one on the upper right of iOS devices.
It's two days old, there isn't a lot of software written for it yet. That's not surprising since most developers just got theirs two days ago. The app experience on iPad wasn't great at first either, remember all the double sized iPhone apps? The focus of the Fire isn't apps, it's Amazon's content. Remember when it was announced and people were guessing whether or not Apps would even be allowed?
It's easy to use, arrives configured (and personalized--the Fire knowing who I am when I opened it was a lot better than the "plug into iTunes" message my iPad(s) came with) and seamlessly hooks into the Amazon ecosystem. It's exactly what I expected.
Update: I just tried the PDF reader, by emailing a document (again, try that on an iPad). It worked great. I added the email address to my address book and now can send docs to my Kindle without having to think. Lovely.