There are occasions when the signal doesn't match the the expectations. I haven't seen this for any HD content myself, but I own a number of DVDs that aren't done in "true" i.e. anamorphic widescreen so when played on an widescreen HDTV you get black bars on all sides. In this case it's nice to be able to tell it to blow it up and fill the screen. (i.e the video is encoded as full screen, including the back bars for widescreen)One review complains that you can't choose the aspect ratio for HD input, only for non-HD input. I don't understand why you'd need that; is that so you can decide how to handle 1080i input on the 720p display?
The LG has the best contrast ratio (1600:1, compared to 1200:1 for the Sharp and 800:1 for the ViewSonic). I don't know much about LG Electronics as a company, but another LG model came up #1 in Consumer Reports' last review of 37" HDTVs.
FWIW LG used to sell their products under the Goldstar brand until recently... Perhaps that name is more familiar.
EDIT: Also, note that some functions may not work as you might expect. My Samsung LCD for instance, has PIP, but it only works for OTA TV signals and not the composite, VGA or HDMI inputs.
Finally I would suggest as others have that 2 HDMI ports should be a bare minimum for consideration of a TV that you hope will last a few years...
B