I'm relatively new to the whole Blu-ray thing; indeed, I resisted buying a Blu-ray player for a long time because I imagined that older movies (e.g., Wrath of Khan) would look grainy and dated in this format, which mercilessly exposes the flaws in older film prints.
So I was pleasantly surprised when "Wrath of Khan" ended up looking, to me, rather good on Blu-ray. It's by no means a perfect print; for example, there's usually some fuzz (or whatever the technical term is) visible on scenes with dark lighting, such as when the Enterprise bridge goes to red alert. But by and large, this print is much nicer than the previous DVD versions.
I compared some DVD scenes to Blu-ray ones to determine whether the upgrade was worth it, and I think it was. Check out the scene when Spock gives Kirk his birthday present; on Blu-ray, you can see all the fine details on that giant globe they're standing next to, whereas on the DVD print it just looks like some glass blob. Similarly, the nebula scenes look much clearer on the Blu-ray.
Some fans seem annoyed that "digital noise reduction" has been applied to the Star Trek movies. Again, I'm no technical expert, but I believe this means that the artifacting/fuzz/whatever-you-call-it has been digitally "painted out" to give the film a cleaner look. This has led some fans to complain that the ST films now look artificially painted over, or waxy, or whatever. I sympathize with this complaint, but I think there's a tough choice to be made here; either studios can digitally "paint out" flaws, resulting in a slightly artificial look, or they can leave the flaws in, resulting in a distracting grainy look. Based on my Blu-ray experiences so far, I favor the noise reduction; for example, the Star Trek Blu-rays look much nicer to me than Fargo, which looks like it was shot through a layer of gauze.
The only real disappointment is that "Wrath of Khan" is only available in its theatrical cut, not the (slightly) better extended cut. It's a shame that Paramount doesn't give you a choice between the two versions; for "Khan," I'd pick the extended cut, and for "Undiscovered Country" I'd go with the theatrical one, but so far Paramount has not released versions that offer the option of switching.
But still, I think "Khan" looks so much nicer as a Blu-ray that I'd rather watch this copy than the special edition DVD. I don't even have a problem with the much-loathed "Delta shield" menu, which may be basic, but has a certain elegance to it...