I think there are two different things here Roger -- each DNG has a very small embedded JPG...basically a thumbnail. RAW + Jpeg gives a big JPEG, which can be read normally. The ideal is when the converter can read the raw file itself -- then you can see things like the histogram, focus point and so on from the photo itself. I think the problem with the Epson is that while now it can show the small preview from DNG files, it still cannot show more extensive information, so if you want to use the big, nice display to check focus, exposure and so on, you still can't do that unless you shoot RAW plus JPEG.
For me, it does not look like the Hyperdrive has a large enough display to make it worth it for me. If I bought one of these, it would only be for viewing of files from the Emotion 54LV, which has a horrible (shockingly, miserably astoundingly bad) screen. It is far worse than the Canon S30 that I had in 2001. The D3's screen is already plenty good enough that I would not really worry about using it. The M8's screen is ok, but not bad enough to shell out the bucks for a device like this.
For me, the backup functions are nice, but a secondary concern.