Oh, I love having it. I love seeing a rainbow of color when I look at a printed page, being able to tell instantly if a word is misspelled (the colors are wrong), and knowing people’s voices by the shape and color as well as the sound. But I don’t think there’s any way someone else could experience it, just as I don’t know what it’s like to have perfect pitch or an eidetic memory or the physical coordination of a figure skater.
I have music playing when I drive because it keeps the part of my mind occupied that would otherwise be inclined to wander. I drive more safely when that part of my mind is kept busy, so I can actually see what’s in front of me and not what I conjure up.
Some pieces always evoke the same experience. Well, at least to an extent. For example, every time I hear a certain piece by Bach, the shapes are the same (much as all paisley prints are “the same”: even though there may be many variations, we call them all paisley), but the colors may vary depending on the instrumentation. Some change quite a bit in color even though the line may remain constant. The line of Pachelbel’s Canon in D is always the same, but sometimes it is one set of colors and sometimes another—there is, after all, a huge amount of difference between strings and brass.
I expect a completely different answer from other synesthetes because the commonality is in the crossover of sensory experiences and not in the specific inner perceptions.