I spend 4-5 hours per day commuting to and from work and, for the last 5 years, have been kept sane while traveling largely by my Panasonic DVD-LS91 portable DVD player. I bought the DMP-B200 as it is very similar to the DVD-LS91: the screen can be positioned in multiple positions, including folded flat against the base for attaching to a car headrest or resting on your lap, which is how I normally use it on the train. It's also reasonably light and compact, far more so than a laptop.
The dimensions are almost the same as the DVD-LS91, being only slightly taller at the rear. Weight-wise it's also comparable and it even takes the same battery. There the similarities end. The screen on the B200 is far superior - brighter and much more colourful. It's obviously sharper as well, being 'high-resolution' WSVGA. Note that this is not the same as high definition, being 1024x600 in pixel resolution so not even 720p HD. In reality this isn't noticeable on an 8.9 inch screen. Playing Disney's Tangled blu-ray disc I was really impressed by the colours and sharpness on display; it's also viewable from quite a wide angle so viewing side-by-side with another person is fine. DVD playback is also very good, much better than the DVD-LS91.
As it's a blu-ray player it does take longer than a DVD player to start up and be ready to play discs but this is only around a minute so not too bad. You can set it to faster start-up but this uses battery power while on standby so will reduce playing time. There is also an SD card slot for viewing AVCHD movies filmed on a camcorder or JPEG photos.
On the downside, when closed for storage the back of the screen feels slightly less sturdy than the DVD-LS91 so I'm not quite so confident of shoving it into my rucksack. It's been fine so far but I might invest in a case. Also it has no internet connectivity so is not compatible with BD-Live functionality on Profile 2.0 discs. The sound through headphones is excellent but through the built-in speakers is tinny and distorts when things get loud.
On the whole though, it's a fine player and when connected to an external TV through HDMI is capable of very good 1080/24p playback with full DTS/Dolby HD audio output. The picture quality is excellent and I'm looking forward to using it for the next 5 years of commuting!