My Take
The best thing about Sumdog is the extremely high levels of student engagement. My students are addicted to Sumdog like it's the hottest video game, and they are constantly discussing their rankings and coin accumulation. Of course, given that they are reinforcing math skills with an adaptive system, I am quite happy with this obsession!
How I Use It
I use this mostly as a homework option, and for reward time in my classroom. Given that my students love it so much, they'll play on the website without any reinforcement while they are at home. So, I don't like to give up too much class time on this. It is a wonderful incentive prize for the students, though.