My personal opinion:
I recently bought a 500M and am loving it. I have a Panasonic VT25 to compare with. I honestly was quite shocked to find that the difference is night and day. The only advantage the Panasonic has is 3D, apps, phosphor trails(although rarely noticeable on 500M) and maybe brightness (ISF day will fix that). I can't stand the Panasonic's severe panning judder or flicker even in 96hz. The AR filter for the Panasonic is also slightly better, however the Pioneer has better overall picture quality because of deeper blacks, more accurate color, greater picture clarity/processing, and a better selection of user settings. The styling is also much nicer than the panasonic.
Right of the bat your will notice the quality of the pioneer...from the box(yes the packaging!) to the remote control, tv stand, user manual and the TV itself. I say go for it, you wont regret it.
EDIT: I forgot to mention...the "fat" 500M is 1.1 inch thinner than the VT25.