I started reading it Saturday at 4:00pm, and finished Sunday about 8:00pm, but i could have read it quicker if not for pesky things like eating and sleeping!
I thought it was really good, but still second to "Goblet of Fire". It just seemed to drag on a bit before they actually got to the school. And there was hardly any quidditch, those are the best bits!
But it was really sad when Sirius died, i was nearly crying, he was my favourite character! I hope they find a way to find him back. I thought it was clever how about 5 different people seemed to be the one who was going to snuff it, it just made Sirius' death more surprising.
It was good to find out some more about James and Lily's past, hopefully there will be more of that in the later books. A lot of people were speculating that maybe Snape had a crush on Lily, which could or could not be true, i guess that might become clearer later on.
Overall, a great book, can't wait for the next one!