According to Wikipedia, synesthesia is commonly misrepresented in literature. I'm not surprised by that, as most high-profile conditions (neurological, mental and physical) get a clumsy handling in the world of fiction, and genuine portrayals are few and far between.
I'm working on the novel I started during NaNoWriMo (no, I didn't get to 50,000 words not even close) and the plot that has been rolling off the top of my head includes a lot to do with synesthesia – both the real synesthesia, and a fictional related condition. The thing is, while I don't want the portrayal of real synesthesia to be seen as totally ignorant, I also want it to be somewhat whimsical and hyperbolic. This is mainly because I like a dash of whimsy in just about everything, but also because the character in question is highly intelligent and imaginative and exhibits a rarer, more curious form of synesthesia: ordinal linguistic personification.
Of course, NaNoWriMo is all about writing without editing as you go, and even though it's over I intend to continue in that spirit, so I'll address these concerns later. Hopefully I won't write myself into a hackish hole.