Last week I finally received my Raspberry Pi model B, and replaced my old HTPC with it. All I can say is I love it! Raspbmc has been able to play every video I've thrown at it without a glitch, including 1080p H.264 .mkvs and SD DivX/Xvid .avis. And the CPU temperature keeps well below 60℃!
The only issue so far are the sticky keys on the Microsoft Wireless Keyboard 2000, which makes typing really difficult. UPDATE: keyboard/mouse replaced by TV remote with HDMI CEC.