I bought the MG77 a few weeks ago. It is my first camcorder so i cannot really compare with other models.
I chose it mainly because i wanted to go HDD and avoid tapes. In fact initially i would expect to do only minor editing so i don't need the superior quality and flexibility offered by avi files produced with DV cameras. The camcorder saves directly in mpeg either on the HDD or on the SD card. Quality is very good for my level and i was very surprised when i saw the movie on my CRT TV: it looked so much better than on my PC LCD screen!
One thing i was a bit disappointed about is that you cannot upload files from an external source (eg PC) back to the camcorder HDD (see my thread here
Uploading files to HDD camcorders. Don't know if it's the normal approach with all the HDD camcorders to protect HDD from external sources or is just JVC that has introduced this restriction on its models.
So be aware no input to the camera, except for mpeg files on SD (hence only small movies).
Teo