This is obviously a heated topic and we're unlikely to come to a consensus in this thread. So please keep in mind that what I say is only my opinion.
I do not believe in the literal six-day interpretation of the creation story of Genesis. At the same time, I think that calling it strictly "metaphorical" is not quite right either. I believe that the creation story is a affirmation of the true, physical fact that God created the entire universe and everything in it with a purpose, and that after the act of creation (but before the Fall) it was good.
So why do I believe this? Let me first explain why I do not believe the literal six-day interpretation. I have two reasons for not doing so. First, there's a mountain of scientific evidence showing much more than six days time between the beginning of the universe and the start of the human race. Second, the two stories in Genesis 1 and Genesis 2, if interpreted literally, contradict each other. (For starters, in chapter 1 humanity is the last act of creation and the man and woman are created simultaneously. In chapter 2 the man is created first, then the plants and animals and everything else, and then woman is created.)
Now why do I believe that the creation account is more than metaphorical? Simply because it fits with what we know, in terms of both theology and science, too well to be merely a poetic device.
In theological terms, many of the Pagan nations that surrounded Israel in Old Testament times did not believe in a purposeful creation. In their mythology, the cosmos were often formed from the swirling blood or sperm of the gods, more or less at random, without any intention on the part of the pagan Gods to produce anything meaningful. Against this empty and purposeless view of the cosmos, the creation story of Genesis is very specific in saying that God created the universes and each level of being within it--matter, plant, animal, and human--with a definite purpose in mind, and that each level of creation was observed to be good.
In scientific terms, once we shake loose from the literal six-day interpretation, the story in Genesis matches up with what science has found.
1. In the beginning God created the heaven and the earth.
In the 19th century many scientists believed that the universe had no beginning; it had always existed and lasted for an infinite amount of time. However, in the 20th century century we have found evidence that the universe did come into existence at a specific time, which means, logically, that there must have been some reason for that beginning.
2. And the earth was without form, and void;
Science confirms that for millions of years after the Big Bang, there were no planets or any other solid matter in the universe. Instead, all matter existed as fluid clouds, which only gradually coalesced into galaxies, stars, and planets.
And the Spirit of God moved upon the face of the waters.
Science has also confirmed that in order for the universe to change into a state where human life became possible, physical laws, constants, and the amounts of matter and energy all had to be set exactly right. If things were only slightly different matter would never have formed into stars and habitable planets. I believe that this truth is reflected in the second verse of the Bible, which seems to describe God moving and acting among primordial matter.
3. And God said, Let there be light: and there was light.
In the early universe there was light emitted from the gas clouds due to high temperatures, even before stars and galaxies were formed. (It's really amazing, when you think about it, that the Bible gets this detail right. Primitive tribes would probably be expected to assume that light came only from the sun and stars.)
Likewise the Genesis account is correct that an ocean covered the early earth before land emerged, that life showed up only after the emergence of land, that plants came before animals, that sea animals came before land animals, that birds came before mammals, and that humans came only after everything else.
Skeptics can, of course, point to some discrepancies, but we need to remember that a detailed technical description of the early universe would have been beyond the grasp of the ancient Israelites, and that the theme of the creation story was to affirm that God created the world with humankind as the endpoint of creation.