So I am curious what is the difference between HD on a BR disk and HD "recompressed" to an MKV file? or maybe a
High quality DiVx formated movie with DD versus an Mpeg2 compressed DVD?
Am I getting my HD space worth by assuming all is well with a 4.8Gig 720p HD movie or a 9.6 gig 1080p movie average size (some are 10-15G though) - BBC Planet Earth for example.
A recompressed HD video will still be superior to a standard-def DVD. However, it will be inferior to the original format. All lossy compressions, by definition, are approximations of the original source. So if you recompress a blu-ray, you're making an approximation of an approximation.... The same goes for a "high quality" DivX knockoff of the original MPEG2 DVD - it's an approximation of an approximation.
We can dive into the whole theory of video compression if you're interested. The main thing modern codecs do is temporal compression. Rather than storing the entire frame each frame, it only stores the difference from the last frame. Every so often there is a 'key frame' that is a full frame to make sure everything hasn't gotten too far out (ever seen a video that is 'sliding'/'smearing'?). This is also necessary for the video to be able to seek. On top of that, the video is split into its primary components (akin to how component video works), it's luminance and chrominance (black/white version and the color data).
Human eyes are much, much more sensitive to black/white, so that 'channel' is compressed the least. Since we are not nearly as sensitive to color, those channels takes the biggest hit, and lose the most quality. Some people can't tell the difference - for some, their displays aren’t good enough to show the difference. Others, especially those who crave high-end systems, can see the blocks and other artifacts created by compression; it drives us nuts.
When you recompress a video, the data is first decoded, and turned back into full frames. When the encoder starts, it now has to not only encode the frames, but also encode all the artifacts that the decoder created. When that recompressed video is played, you now have at a minimum of 2x the visual artifacts. Audio never loses quality, and that should be mentioned, the streams are simply copied over.
"Planet Earth" (new "Life" coming soon too!) is absolute perfect example of a video I would never, ever want to recompress. It was videoed and produced using only the highest quality equipment, and it really shows. Try watching "Planet Earth" on a standard-def television and you'll see it isn't nearly as breathtaking.
For me, I'm completely willing to spend up to 8GB/DVD. I also have my entire audio library in a lossless codec (roughly 2x to 3x the size of an mp3). Even though I have over 10TB of storage on my network, I am not at the point that I would copy all of my blu-rays onto the network. Even with disk space very cheap these days, for me, it would require that I purchase a third NAS or build a storage server. This may, however, be an option for some who integrate their media and storage servers, and build it to scale out as they collect more movies. Approximately 30 blu-ray disks per TB means you'll need a lot of drives for a moderately-sized collection.
As with most things, there are pros/cons to this situation. You need to find out what is most important to you, and your budget. If quality comes first, it may not be viable to host your blu-rays on a NAS, but viable on a media/storage combo server. Maybe you want to recompress your blu-rays and put them on your NAS, but actually put the disk in for the Epics (LOTR, SW, Planet Earth, etc.). Maybe you really can't tell the difference and just don't care - and that's fine too.