So I've been using Linux in my HTPC for 2 months now. All I can say is, I love it. Some features aren't as pleasant to use as they were in GB-PVR, but overall the experience is nice. New functionality that I really like is having a separate workspace that I can access through VNC.
I ended up using Ubuntu server edition as the base. The main front-end is MMS (My Media System). TV side get's handled with VDR and Xineliboutput-plugin. All and all the system is very stable and solid. There were some issues with the video quality, mainly due to deinterlacing. That issue got solved after 2 weeks of trial and error. Now the video is perfectly smooth and fluid.