Free city tours have been getting a bad rep in recent years. Many people just don't vibe with the organized and "touristy" nature of these tours, but, with all due respect, these tours were made explicitly for tourists, and as such, they're great for getting to know the main attractions of the city. They're surprisingly efficient and informative, and you get to talk extensively with a local guide who knows all about the place.
There are, however, six more words we would have to add to our original three: don't forget to leave a tip! Seriously, those guides work hard for their money, so why not?