Dear Thanks, You and your friend are both right. They can be a monumental suck-up. Or they can be a heck of a way to strengthen your chance of landing a job. Well, he got caught giving counterfeit thanks, but you don't have to.
Insincerity is the surest form of sucking up.
To avoid insincerity and ensure that you hit the tone just right, have a few people read the letter before you send it. Show them that you listened during the interview by summing up the most important aspects of the job, then briefly remind them why you're the best person to take on those responsibilities. I recently bumped into an acquaintance I hadn't seen in several years. While we chatted, I reminded him of a story he'd told me when we last met. The guy was blown away, flattered and impressed that I'd remembered his story.