We have some AVCHD files (MPEG 4, 1280x720 H.264, AAC) that we have had a world of trouble editing with Premiere Elements. The main problem is that the audio and video get out of sync. Reviewing this question on the Adobe forums, the likely issue seems to be that Premiere Elements doesn't import some MPEG 4 formats correctly and Adobe recommends transcoding.
We have Sorenson Squeeze 7 in-house (we're using simple iMacs), so my first thought is to simply transcode. The question is what format is best from an editing (and preservation) standpoint. I am currently trying the AVI "NTSC DV" format, since I =think= that's what we get off mini-DV tapes, but I would hate to be throwing away resolution or image/sound quality if there is a more recent, non-lossy format that we should be using. I'm also quite open to upgrading to Sorenson Squeeze 8, purchasing/installing a different transcoder, and/or going to a professional video editing platform--although the latter seems to be a waste. We pretty much exclusively generate flash and MP4 clips from our source files--simplest editing, rarely use any sort of effects (and nobody here knows video editing more sophisticated than what can be done with iMovie--ie., home movie production).
So, my primary goal is to generate reasonably good clips today, and to be saving master files in the best, stable, long-term "archival" format that is accessible without specialized equipment.