One thing I will say is this; never take my word for anything I write here. Have a think about it, question it, do your own research into the topic, ask yourself if it sounds likely and what mechanism might be behind it. I am more than capable of making mistakes, mistyping, or getting completely the wrong end of the stick. I would encourage you to take this approach whenever anyone tells you anything; always try to think critically, you'll learn a lot about the subject at hand but also about yourself and the people you interact with.
I mentioned earlier that being a synaesthete can have it's advantages, let's look at a few examples. Look at the picture below, try to look at the picture on the left first. Not very interesting is it? Just a whole bunch of 5s randomly strewn about. The picture on the left is a little more interesting, it is identical except that colour has been added. All of a sudden it is very apparent that it isn't just a mass of 5s on the page, there are also a few 2s thrown in. You would have eventually spotted that in the original picture if you'd had a good look at it, but it required only a fraction of a second in the next image (admittedly, as red and green were used, colour blind people will have struggled with this one). People with grapheme-colour synaesthesia pick up on the 2s far faster than the rest of us do.
So what practical applications are there for this? Look at this next picture below of some simple sums. We can all do these, they're at a pre-school level, but children tend to grasp colours before they do numbers and those children who also have grapheme-colour synaethesia will have an extra level of association to latch onto that will likely propel them to the head of maths class. If they learn to harness this to it's full potential then maybe that's one way how we get people like Richard Feynman. You may also imagine how Liszt saw the colours of the notes of his composition rising from the orchestra; if you also had an idea of the colours you wanted to invoke as you composed then this could potentially be an extra tool with which to fine tune the piece as compared to those of us who have to just listen very carefully.
The first reference I can find to synaesthesia was in 1880 by the legendary Francis Galton, so we have known about this for quite a while and as awareness of the condition grows it is now thought that as many as 4% of people could be affected, which sounds like a lot, but many don't even realise they are unusual. The question boils down to this: Is this an entirely new type of phenomenon in the brain or merely an extreme form of something very typical? This is a question that crops up time and time again in neurology and hopefully in many cases we will be able to resolve it as we slowly get a handle on what baseline brain activity really is, if there turns out to be such a thing. One final example, look at the picture below. One of these shapes is called Booba (stop giggling) and the other is called Kiki. Decide for yourself which is which. I know which way I went and I can be fairly confident of which way you will go; the jagged shape on the left will be Kiki and the more rounded one on the right will be Booba (stop it). That's how 98% of people assign them.
So is this a very low form of synaesthesia that we all share? Is it a phenomenon that is unrelated but gives a similar sort of outcome? Is it more of a cultural effect to do with the shapes our mouths and lips take as we pronounce the words Kiki and Booba (see me after class) and the way we associate them with the symbols in the picture? The answer is: we simply don't know. But I do know how we'll find out; through careful observation, rigorous application of critical thinking and leaving your preconceived ideas at the door: through science.
All images used with permission under the Creative Commons License