I just found this thread today and wanted to add my experience with fish eyes. I haven't had eyes from animals other than various kinds of fish, squid and octopus. I'd like to try and I wonder if they taste similar or not at all.
There is a tiny fish here called cornalito that is around 6cm long (2 inches) and is eaten battered and fried instead of, or in addition to french fries. It's usually served with other meats that are battered or breaded and fried, such as thinly sliced beef, chicken, or squid rings. Battered and fried onion rings and breaded and fried mozarella cheese are also popular things that are served with them.
Cornalitos are traditionally eaten whole, with the head and everything. Lately (in the past 10-15 years) some people have began removing the head and the intestinal tract to make them less bitter, and I've heard some people say that it's the eyes that make it slightly bitter.
In any case, I haven't had those little fish raw because I couldn't find them fresh, only frozen. But I've had other, much larger whole fish. Such as bonito, which raw, for some reason, tastes a lot like canned tuna. Or another one called besugo which is similar to a red snapper. (I'll post a picture of the besugo, which is the first one I had whole, this one weighted 1.25kg (2.8lb) before de-scaling and was 6cm thick (2 inches). I took the picture before de-scaling it)
So after having the skin, meat and some of the internal organs (which are super bitter), I tried the eyes. I was expecting them to be horribly bitter, but wanted to have them anyway for the nutritional value. Because fish don't have eyelids, it's really bizarre to be looking straight into it's eye as you're carefully carving with a knife and pulling it away with a fork to prevent popping it and making a mess. The psychological effect of doing that while planning to put it in my mouth and chewing was a bit hard to prepare for, but then I just didn't think about it and did it. It seems that we have a tendency to associate open eyes staring at us with something being alive and watching us (I hear the same works on birds, and that some scarecrows now are just a big eye). It can be hard to tell yourself that that's not the case and that the thing is dead.
To my surprise, it was the best tasting part of the fish by far. Like others here said, it's really sweet, almost like a raspberry. And even when the fish is not 100% fresh it still tastes good (just not as good). The fatty tissue that hangs around the eye socket after you remove the eye is also very tasty (some of it sticks to the eye as you pull it out). The rest of the fish heads I could not manage to eat, so I throw them away.
Eyes from squid and octopus are pretty good also, just not as sweet, and more chewy. And they don't have the same crunchy membrane that fish have (or it might be that the fish eyes I've had were much larger)
My eyesight has improved since I've been on this diet (3.5 months so far), but I can't say if it's due to consuming raw fish eyes or everything else. I haven't had much fish eyes by comparison to everything else that changed in my diet.