I've always used libavcodec because my computers have always been fast enough to decode H264 without problem.

In the early days when there was only SD H264 encodes, my Athlon 64 handled that no problem. By the time HD became popular, I had already upgraded to Core 2 Duo, which decoded that without breaking a sweat. My current computer can handle 1080p H264 using libavcodec with only around 60-70% CPU usage, so it'll be a while before I need a faster decoder or a faster computer.