Yes, I think there are still a few issues using it with Win 7 for some people, but with Win XP, I'm not having any issues to speak of. The DSPlayer pulls my CPU useage on 720P .mkvs down to under 8%, not too shabby.
I was using MediaPortal for many years and recently switched to XBMC and I couldn't be any happier. I'm running XBMC 9.11 with the Showmix 2.5 skin and everything is perfectly stable. I have XBMC configured with Zoomplayer Professional using the MPC-HC standalone video decoder configured for DXVA as an external player. I'm using Girder with a USB-UIRT for control of applications including standby and resume function. I also configured MPC-HC as a secondary external player for the very rare situation I require DXVA and subtitles.
I also have BOXEE installed with the ability to easily switch between BOXEE and XBMC with with a single button on my remote. BOXEE is nice for web content but it's far from stable and I wouldn't want to be using it for a front-end to manage my local media server content.
I am setting up my third HTPC for personal use & will be using XBMC under Linux. Hardware acceleration is available , VDAPU, and works great as long as you have an Nvidia based GPU and more recent drivers installed which is a piece of cake.
Previously I used Mediaportal extensively but in between the long development update times & having to rebuild the database one too many times I went out & tried a few alternatives & XBMC stood out.
Setting up & using XBMC under Linux is super easy as long as you use a supported Distro such as Ubuntu & it's derivatives. For my build I plan on using Linux Mint 8 and run XBMC in Stand Alone mode.
Boxee is nice being based on XBMC source code with lots of additions but I don't find it stable enough yet.