Well, I just made a comparison between the BBC Planet Earth DVD series vs. the Blu-ray discs. The difference is HUGE and the blu-ray images just pop out of the screen with obvious increased level of detail - just look at the images of the thousands of birds migrating. On the DVD the birds just look like triangles moving across the screen, whereas on the blu-ray each bird can clearly be distinguished as having the shape of a bird and not being just a little white triangle.
In addition to that the new high definition audio soundtracks (like uncompressed 24-bit/96kHz, DTS-MA, etc.) make a huge difference on a capable surround system - even that alone makes the price of admission worthwhile.
Just my 2c...