I bought a JVC Everio GZ-MG21 last year. I couldn't get the included software to work at all - it reported "A necessary Library is missing" - the helpdesk was useless.
Anyway searched the WWW and the consensus seems to be the included Mac software is poor and some users report that it renders the pictures badly (but maybe they used it incorrectly).
Of course, you want to convert without loss of quality. My Web searches indicated you get the best results by using "Mpeg Streamclip" which is a free download, but you also need Apple's "MPEG-2 playback" which is $19.99 (or 14.99GBP for me) - inc. taxes, no delivery charges - you download it.
That worked great for me.
Here are the notes I made for myself:
FastImport of Everio Files to iMovie
1. Create a New iMovie Project for DV (this matches the Everio quality)
2. Create an alias of the Media component of the iMovie Project
in the Movies folder
3. Use MPEG Streamclip Batch Convert files to DV (DV25) 720 x 756 aspect ration 4:3 (16.9 ?) deinterlace at Zoom 100% X/Y=1 Center 0,0 (not selecting FrameBlending/BetterDownscaling/RsampleAudio/SplitDV/Cropping), putting (via an alias) the results in the Media component of the iMovie Project
[Note: 4:3 is the normal format for the GZ-MG21.]
4. Close the iMovie project and re-open it - iMovie should then report the DV files are in the iMovie trash.
5. Move (drag) the files from the iMovie trash to the timeline or clips.
6. Save the iMovie project!!
This method avoids re-encoding the files twice. This is almost as good as iMovie supporting the .MOD (that is MPEG2) format. Otherwise if you create the .DV files first and then reimport them into iMovie you end with two copies - one in the iMovie project and one outside - taking up twice as much space.
I am not sure the deinterlace is needed, but seems to avoid jaggies on individual frames - though I think it increases (doubles?) the .DV file sizes.
Other things worh noting.
The 4:3 Movies from the camcorder are 768 x 756, but the 'DV' setting says 720. Dont be fooled by this: the re-encoded Movie is 'DV, 720 x 576 (768 x 576), Millions DV, Stereo, 48.000 kHz' with 768 x 576 pixels (Actual).
You can change the extension to the .MOD files to an MPEG-2 Playback File Type (.mpg, .mpeg, .vob(2), .vro, .m2v, .m2a, or .m2s) and open the files in QuickTime Player, but in that case you seem to loose the sound (at least I didn't get to to work). MPEG Streamclip produces a .DV file with the sound included.
Summary: I can't understand the reports of poor video quality for these cameras - it seems very good to me. Maybe there is an issue with the software supplied which then ruins what the camera captured. JVC would best advised to ship software that allows import into iMovie without loss of quality.
Note: When I posted this before, someone commented that you can't "import without loss of quality" because MPEG2 compresses and therefore loses quality. This is nothing to do with the import process and will happen when the file is recorded. It should be possible to import the MPEG2 file without loss of quality (compared with the MPEG2 recording). The MPEG2 quality will depend on the degree of compression used when the original recording was made.