As shown on the picture above, for a creature with composite eyes, the Earth still look like the old planet we still know but the details are poor. For an insect, the important point is probably to distinguish the flowers from the landscape and your hand (or predator) before you hit him. It is simple and well, it does the work for them but seriously, any large creature would be greatly disadvantaged by this type of eyes.
No slit at all; eyes appearing like a crystal ("crystalline" eyes)?
Think about the pernese dragons. How are they able to see something? In fact, the best explanation is there is no retina but instead the light receptors are distributed homogeneously in the eyes. This means a photoreceptor near the surface can see the object far away and you go deeper into the eyes, the photoreceptors are able to view objects that are closer.
This would also means that the eyes are always focused no matter the distance of the object (I admit this could seem very strange for most of us, but imagine a photo where the close objects are as sharp and crisp as the objects far away).
However, this do not give the same precision as a retina since the dragon cannot have a dense quantity of receptors without blocking the light; a dense quantity of cells would block the light for the layers deeper in the eye (to some extant, in fact, cells are almost totally transparent).
As said above, the advantage would be to be to always have the close and far object focused at the same time and having a great deep perception even with a single eye. I explain, for human and some other creatures having a binocular vision, this is extremely useful for evaluating deepness but if your eyes are focussed on every distance all the time and if your brain is able to tell which signals from which "layer" of photoreceptors, then the brain no longer need two eyes to know the "exact" distance between the creature and an object. This would allow the dragon to have his eyes slightly more on the side and have a greater angle of view without sacrificing their ability to determine deepness.
Another advantage would be to increase the angle of the eyes, bird have their eyes on each side and human right in front. We humans with our eyes squarely in the front of our heads, can see about 180 degrees, but we need to direct our eyes into the direction of the object to have a clear picture. The crystalline type of eyes would be focused not only for object of all range but also in all directions, this means once you come into their angle of vision, it is like if the dragon was looking directly at you all the time while we cannot describe very well a person standing in the corner of our vision.
The main disadvantage would be their inability to have one point where all their light receptors can be gathered. Owls for example have a very dense retina and this allow them to see small creatures (such as mice) even if they are very far from them. In a crystalline eye, this situation cannot exist, they can have a good visual acuity but not as high as the animals that are renown for their vision. Does this means a dragon with such eyes cannot have a great vision? Absolutely not, again, we humans have a vision angle of 180 degree and about 140 degrees of binocular overlap. Dragons with crystalline eyes could have an angle of vision around 220 degree (i.e. the eyes slightly on the side) and an overlaps of 110 degree. The object inside the overlapping degrees of both eyes would greatly increase the resolution of the object while the remaining angle of vision would be less defined (but much better defined than the visual acuity of the corner of our eyes). The degree where the vision of the dragon overlap would be equivalent to the area of sharpest focus and would be obviously much greater than our.
Glowing crystalline eyes?
Glowing eyes without slit are nonsense, I have see some trying to explain the phenomena but their explanation contain several flaws that make the theory highly questionable. Let face it, this is often used to show good/bad guys in manga and other cartoon but this remains very unrealistic. In fact, if the creature had an iris, it would be the reflection on the iris that would give the impression of glowing eyes. This is where the idea came from but as you all surely noticed, the "glowing" eyes of a cat in the dark do not really glow; it is just a reflection of the light. Now, imagine now that your pupil would glow, you would literally blind yourself by the light emitted but also by the reflection of this light back to you. The best example I can give is at night when you are inside your house, if you look outside with all the lamp turned on, the glass in the window will acts like mirror. In other words, if you need to look outside, it is easier when there is no light inside. Same thing for the eyes, if you have a light source inside the eye, this would 1) blind you by activating the photoreceptors around the light source, and 2) your own cornea would behave like a mirror and reflect you back the light you emitted thus blinding you even more. In a crystalline eye, the light emitted by the "crystalline" liquid would trigger photoreceptors that are surrounding the light source thus completely blinding the creature in the process.
All this to says, that artistically, this give dragons and fantastic look but scientifically there is so many problems generated by this type of eyes, that it would be a lot of trouble to gain no advantage over the other types of eyes...