The total opposite to the usual post here - confirmation of better performance and encouragement for the developers.
I'm pushing the envelope a bit working with AVCHD 24mbps files and 1920x1080 60i from the Canon HF11.
I experienced several problems with the Ubuntu packages from sunab that were built in July: artefacts and jittering in clips, skipping back and forward when seeking, and stuttering when doing real-time previewing.
I've built the latest development snapshot packages for ffmpeg (0.5+svn20090820), MLT (0.4.4+git20090820) and kdenlive (0.7.5+svn20090818) today for Jaunty (detailed in the "Installation from binary packages" forum).
Much of the artefacts and jittering has gone, skipping when seeking is much improved, and real-time previewing is much smoother.