Sort of off-topic: the other day I needed to convert an AVC/h264 file in an .avi wrapper that was output from an old screen capture software for a series of tutorials. It needed to be editable, and easily iDevice compatible. I can tell you I tried every F-ing thing I could think of involving AVIsynth, FFmpeg, VLC, QT 7 Pro, VirtualDub/Mod, FCP, Premiere Pro and nothing would convert it. It only played through VLC.
We are talking 10 hour-long .avis, at 5 GB each. Then I found this AiSeeSoft Total Video Converter software, sucked it up and paid $40 and it's the best switchblade conversion tool I have ever used. It will ingest anything and batch output any format, with about 90 presets of devices and codecs. I think it even does Prores on PC, it definitely will output 10 bit DNxHD with alpha support, and it uses CUDA / OpenCL. I converted 2 hours of AVC into .mp4 in about 20 minutes at a 4:1 compression. For example it will even demux .mkv matroska files. Definitely worth a look if anyone runs into these issues.