I spent a good chunk of the day developing new materials for Arki's EYEdeas 3+:
The materials that come with the set are on the left of the screen, and the mine are on the screen's right. (Or, to confuse matters, Arki's are on the model's right and mine are on the model's left.)
The eye models themselves are quite good. They're very detailed with all the geometry you need to make extremely realistic eyes, even to the point where you could conceivably create a photo red-eye effect or realistic cataracts. And the textures are OK. Nothing that really knocked my socks off, but a good solid collection of staples in the Naturals collection I bought.
The materials, though, I was not very impressed with. I like my materials to be based on real-world physics. Oh, I suppose there's something to be said for the "what looks right however you get there" method, but the problem with that is what looks right under one set of conditions often looks like crap under others. What's based on realistic physics will continue to look realistic however the lighting conditions and other scene elements change.
So, I went and built a physics-based shader for it. I especially like that if you look closely in the (model's) left eye, you can see a reflection of the environment. It's a bit exaggerated because it's an image map on a self-illuminating environment sphere. In other words, normally you'd see a reflection of light sources/bright objects only in an eye, but in this case I've turned an entire city into a light source, so more is reflected than usual. (If you're wondering about the dual "hot spots", I'm using a light set with two lights. Actually it's four lights, but only two are in front of the model.)
The good news out of all this work is that now I have shaders set up for both the set with original mapping, and the set that takes Gen 4 maps, so going forward it's just a couple of mouse clicks. (I can't say enough good things about Semidieu's Shader Builder. Setting up the original shader takes a while, but it saves me so much time and sanity in the long run. I don't know what I'll do while waiting for an update if it breaks in Poser 9.)
Now, the question you may have is, does the difference in materials really matter when you're not close enough to see the model's eye boogers? Depends. Like I said, "looks right however you got there" looks great in some environemts, and not so great in others. And the original materials are probably good enough for government work. But then again, if you're just going for "good enough", you're probably not bothering with a replacement eye model to start with. If you're going to go to that trouble for extra realism, it seems worth it to go all the way. Especially since eyes are such an important part of a portrait.
