To compliment someone on their penmanship, you can say "Tiene bonita letra".
Now, if you've been reading my blog long enough, you already know this post isn't just about complimenting someone on their handwriting.
Ever have someone try to set you up on a blind date, and when you ask "What's she like?" And the answer is, "She's got a great personality". Well, we all know what that means.
It should come as no surprise that this happens in Mexico too. So if you ask the question "¿Como es ella?", the answer just might be "Tiene bonita letra" - The equivalent of "she's got a great personality".
Using this phrase in this manner is very much a Mexicanism, and you use it when you're talking about a member of the opposite sex - sexo opuesto.