when should you consider read replicas for your database?

Hey everyone, I've been mulling over database scaling for a bit, particularly for apps that get hammered with reads. We've gotten by with just one main database for a while, but with more users, all those read requests are really starting to add up. I'm wondering what your experience has been and how you figure out when it's the right time to add read replicas. What are the main signs that tell you replicas are the way to go, instead of just tweaking your queries or beefing up the main server? Any hidden problems or particular tricks you've learned when setting them up?