Well, I've now completed my review of Enterprise Season Four on Blu-ray - and I can at last give a hearty recommendation for a genuinely worthy upgrade from the previous DVD release.
Sharpness and clarity are significantly increased, with vibrant colours and pleasing black levels. It's certainly not perfect - patches of softness sporadically appear throughout - but that may have more to do with filming techniques.
The DTS Master Audio is solid, with some excellent separation and clear, easily discernable dialogue from the centre channel. Not spectacular sound, but this is an early 2000's TV show and is pretty good for what it is.
I've read a lot about the switch to digital giving a more "studio like" appearance to the show - but I'm not really seeing it. It's all graded quite nicely, and the dreaded "soap opera effect" is minimal to non-existent IMHO.