Short answer: if done right there is no damage to your paint. You can use lighter fluid, goo b gone or any solvent to remove the residue and no, the letters cannot be reused.

It's an easy 15 - 20 minute job using a hair dryer to soften the glue and fishing line to slide under each letter to remove it.

For more details do a search, it's been discussed here many times. Good luck.