Mine has always been very visual. Sounds, letters, ideas, and tastes often present themselves with colors, textures, shapes, and sizes when I think about them. I don't experience synesthesia out-in-the-world, only in my head. In other words, when I read a book the letters on the page don't have any color, but if I think about a letter, or listen to a song, very often it'll have a color or a size. This happens most strongly with music, generally speaking. But I also remember that I almost got in trouble around the age of 3 or so because I was explaining to an older girl what size and shape various swear words were.
This also seems to be rooted pretty deeply in my thought process. It's very difficult to watch your own mind think, but I've noticed that a small percentage of my thought is a stream of incomplete sentences and a bigger part is images related to those sentences. Another significant chunk, however, is just weird shapes moving and changing and banging into each other. I might be trying to think about something like "justice" or "anarchy" or "rationality", and what I see is a strange chalky prism morphing into a sphere and then shooting off to the right, leaving a colorful trail of liquid smoke. It doesn't make any sense, but I just somehow know that what I'm seeing is a thought related to justice.
It's like the machine language of my mind is a polychromatic rainbow, and there is a synesthetic compiler in my unconscious which has to paint speech and thoughts before my brain can do anything with them.
So remember that next time we're talking about something face-to-face. My sentences and thoughts may come out articulate and orderly, but between my ears it most likely looks like what would happen if Jimi Hendrix, Walt Disney, and Wassily Kandisky took LSD for a week and then were locked in a room with painting supplies, the collected works of Euclid, and high-end computer software after being instructed to make a video game together.