I have a few of the visual forms of synesthesia, which I've read are the most common. I see colors in letters and numbers, and I also see music as colors, and all different sounds for that matter. Each letter and digit has a distinct color (capital letters can differ from lower-case), but when grouped together in words or bigger numbers the colors come together, but in a really weird way. The only thing I can liken it to is relatively thin sheets of colored glass, overlapping each other, only I can see every color distinctly, and the mixing of colors is vague at best. It's more like a 3D grid of square pieces of colored glass, that I see equally from the side or at an angle just as much as from the front - perhaps more so.
I wonder sometimes if this is part of why I get so overstimulated so easily. When I hear any sound I see colors, and so combining that with "real" visual stimuli would automatically be more overall stimulus than non-synesthetes get. Then imagine if the "normal" world contains bright lights, the sounds are louder, people's talking voices become a big jumble, smells are strong, etc...
On the other hand, there are parts of the synesthesia I love. When listening to music, I don't need to do anything visual. If I am listening to a symphonic piece, I can simply close my eyes and watch the colors as well as visualizing the instruments in my head, and watching notes go up and down the sort of "maps" in my head -- I "see" things going up and down in scale-like ways, and how I see it may differ depending on the instrument. For example, with a piano, because I used to play, I'm usually seeing the actual piano, the black and white, picturing how someone's fingers might be moving up and down the keys. With something like a trumpet, though, I'm just seeing the notes move upwards or downwards, mostly independent of any other visual stimuli.
I also think that digits and letters having colors is something that has made memorization easier, particularly when it comes to things like spelling. It also helps me figure out which number I'm remembering incorrectly if, for example, I'm not sure whether a date was one year or another. Eg: If I wasn't sure if something happened in 1973 or 1974, but know the number had no green in it, I know the correct year was 1974. I also think it's the synesthesia which has probably helped me develop perfect pitch, stemming from my instinctual knowledge of one note, that note being middle C.
So I am one of the 4 or 5 people out of one hundred experience some type of synaesthesia. (The color of 5 isn't quite correct though... it also involves black, and some very dark green as well. Weird synesthesia, right?)