doof is different from other sites that it has strong community features, you can interact with others, you can challenge community members and participate in leagues.
You can play basic games for free but for playing multi player games you need doof credits. doof credits can be purchased by using Paypal or Credit cards.
User interface is designed all in flash and it looks impressive, you can even customize the colors of panels to suit your taste.
On the other side doof is slow because of its all flash platform, and its pain sometimes to wait for the game to load. It would be good if they can improve the site responsiveness. Its business model is based on members buying credits to play games, i am not sure how far this will help them to sustain in this crowded free games market. Overall the site is impressive and you can spend your free time playing some nice games.