DD has this. None of the rest of us do. She has the "quirkiest" brain in the family, in my opinion--shades of ASD, SPD, ADHD, and prodigious/photographic memory, along with depression, anxiety, and great creativity (visual and writing, but this extends to almost anything). No official dx other than anxiety/depression, though, and definitely no LDs. Her school performance is very even across the board.

Interestingly, some of the synesthesia seems to be fading as she gets older--she reports that that the letter-color associations are sort of going away, though number/color remains as strong and consistent as ever. This is apparently not uncommon.

The net has tons of interesting research on this. It seems to be hard to know the true incidence, but I've met quite a few people with some form of it since I learned that DD has it.