The bottom line is the ENT BD's are better than the DVD's in both sound and picture quality, and I would recommend purchasing them as your best available option if you don't own the series and are considering a home video purchase.
However, big caveat, the increase in quality over the DVD's is often fairly minimal, and indeed, in certain scenes - marginal.
TOS on Blu is significantly
superior to all antecedent home video releases, and TNG literally shits on the DVD's from a great height with a revelatory, night and day, enhancement in quality.
If you've been wowed by TOS, or blown away by TNG, you are definitely not
going to feel the same way about ENT on Blu-ray - and are in for disappointment.
Upgrading to Blu from DVD is for probably for the more hardcore Trek/ENT fans only, or perhaps those who may notice moderate improvements on larger display devices and home cinema sound systems (that's me)!
If you're the former, I would advise tempering your expectations, for as mentioned above, improvements are sometimes subtle.