There exists a great deal of variation between synesthetes in terms of what they experience.  Even among those who experience the same modalities, there are large qualitative differences.  The following passage (3) compares the experiences of a family of colored-letter synesthetes:

“A, pure white, and like china in texture.
E, red, not transparent; vermillion with china white would represent it.
I, light bright yellow; gamboge
O, black, but transparent; the color of deep water seen through thick clear ice.
U, purple.
Y, a dingier color than I.
“The shorter sounds of the vowels are less vivid and pure in color.  Consonants are almost or quite colorless to me, though there is some blackness about M…
“Of my two daughters, one sees the colors quite differently from this (A, blue; E, white; I, black; O, whitey-brownish; U, opaque brown). The other is only heterodox on the A and O; A being with her black, and O is white.  My sister and I never agreed about these colors, and I doubt whether my two brothers feel the chromatic force of the vowels at all.”
 

Cytowic’s first subject, MW, “tastes shapes” in the following way:
“When I taste something with an intense flavor, the feeling sweeps down my arm into my fingertips.  I feel it—its weight, its texture, whether it’s warm or cold, everything.  I feel it like I’m actually grasping something” (p. 4).

He described Angostura bitters as “an organic sphere with tendrils.  The shape feels like a living thing, see, which is why I say ‘organic’.  It’s round, but irregular, like a ball of dough.  The quinine…felt like polished wood because it was so smooth… but this sample is bitter in a different way.  It’s hard to describe…this definitely has an organic shape.  It has the springy consistency of a mushroom, almost round, but I can feel bumps and can stick my fingers into little holes in the surface.  There are leafy tendril-like things coming out of the holes, six of them….here are the strands.  A little thread.  It gets bigger, like a rope.  If I pull my hand along one it feels like oily leaves on a short vine.  I guess the whole thing feels like a scraggly basket of hanging ivy” ((1), p. 64-65).
 

One individual in particular, A.R. Luria’s famous subject S, had a very severe case of synesthesia, such that he was not even aware that any line existed to separate the senses.  He had an amazing photographic memory, but was so overwhelmed by myriad sensations that they impaired his ability to function normally.  In his words, “What first strikes me is the color of someone’s voice…He has a crumbly, yellow voice, like a flame with protruding fibers.  Sometimes I get so interested in the voice, I can’t understand what’s being said…Should another person’s voice break in, blurs appear.  These creep into the syllables of the words and I can’t make out what’s being said.” (12)