1080p H264/x264 mkv are being played flawless on G5 2.5Ghz. 720p h264 mkv are okish on PMac 3.1 G4 1.6Ghz. Buffering when skipping forward/backward is a problem but I assume that is caused due to the very slow and old IDE HD interface of my main system. Streaming over lan from a NAS is ok too. Highly compressed MP4 are usually proved to be a problem on the other hand.