Thanks for starting this topic! It will be a great resource.
JMA, I'm probably the one you were listening to when you tried the ND filter... of course, if you prefer the image without it, that's all that counts.
However, be aware that using the ND filter does not mean you have to sacrafice color saturation, shadow detail or anything else - the 5700's excellent user controls allows you to bring out every nuance of the image, filter or not.
Also, the filter does much more than improve blacks - it also brings down the brightness to a far more managable level. This is good for two reasons.
Ambient light: my living room is PITCH BLACK, which is what you want for the best possible image. However, the 5700 is SO bright, light bounces off the screen into every part of the room and creates ambient light everywhere! All this light then gets reflected back onto the screen and washes out your picture (to a greater or lesser extent based upon your viewing environment). And I'm not just being anal, I swear the light coming out of this thing is so strong the reflected light lit up my house like I turned all the lights on! Using the ND filter helps this enourmously.
Rainbows: For better or worse, I can see them. As I stated in my long review, I don't see a lot of RGB breakup, but the subtle strobing in high contrast scenes (such as candles in a dark room or streetlights at night) are not just distracting but have actually cause me eye strain! The super bright image makes this effect a lot worse. In cutting the light output, the ND filter reduces the rainbow effect to a very acceptable level.
I know there are people out there who just want the brightest possible image, and that's fine. I used to have all my TVs and computer monitors really bright until I learned about proper callibration. I remember when I callibrated my first TV, I thought the picture was suddeny way too dark... but within a day or two I got used to it and noticed how much crisper, sharper and more lifelike the picture was (not to mention how much longer the tube would last).
The same is true of the 5700. Out of the box, you'll be amazed how bright and snappy the image is. But (at least for me) after just a day I found it to simply be way too bright. After popping in the ND filter and recallibrating, I was suddenly in awe of the image. Colors, contrast and black levels are nearly at a CRT level (only the very darkest scenes show blacks as a dull grey... but it is VERY dull).
To each his own, yes... but I would recommend that anyone who owns a 5700 (and probably a 7200) who wants the best possible picture should try it.