I find that when I'm interviewing people, I need to sit off to the side of the camera and still be able to monitor the shot (they squirm and mess up my framing).
I bought one of these just for that purpose. Here's my story-
One day, long ago, I was looking at portable dvd players and they were all crap. The screens looked like hell. Then, I saw this one. Ta da! The LG DP781. The screen looked amazing! Sharp and bright and crisp and clear. I couldn't believe it. IT EVEN HAD AN A/V INPUT so I could use it as a set monitor (composite in only). I didn't end up buying it then, but when I got my 35mm adapter I needed an external monitor. I remembered the DP781. I sniped one on ebay and had it at my door for $72!
It has a FLIP function that works perfect for 35mm adapters, and it has a screen that will rotate and lie flat so the whole thing could be mounted on a camera with some kind of bracket. Sound good? Well, I soon realized that when plugging into the A/V input the picture is CRAP, and you only have two choices for aspect ratio. 16:9 or OFF. Since I shoot in letterbox, it doesn't work very well, not to mention the screen controls are only brightness and picture.
The short story is, if you have no money, you can have a monitor that comes with a battery for less than $100 that will work (in 16:9 only). Is it better than my DVX lcd for focusing? Yeah, I guess so. Sorta.
Also, there are a ton of these things on ebay sold as "broken Items", which makes me wonder how long this thing will last. i have yet to find out.
If you're ever buying a portable dvd player though. This one blows everything else out of the water for picture quality.
We needed to buy a portable DVD for my kids school work anyway, so I figured I might try to get one that could double as a monitor. Sounds like it might serve my purposes. I shoot mainly in 720p so it might be okay. Mainly, I just wanted something that would help me keep my mind on what's on the screen while I'm away from the camera doing interviews.