I have been testing out Flash playback of h.264 video.
The source is DV 720x480 48khz audio. Using Premiere CS3 I compress to h.264, 720x480 VBR with AAC audio.
Playback on my machine using VLC media player looks ok with audio in sync.
Playback using Quicktime Player has audio sync issues (obvious lip sync).
Playback off my web server using an embedded Flash player looks good with perfect audio sync.
I don't know how you can make a general statement like that. Quicktime 7.5 handles H264 just fine. Just take a look at any of the HD trailers on Apple's site.
Certainly there are workflows and compression apps that can have issues but that's not a Quicktime playback issue. If you have a computer that can handle the decode Quicktime 7.5 playback of H264 looks very good.