database sharding, when do you really need it

So, I've been chewing on the idea of scaling databases lately. Our one PostgreSQL instance is really starting to creak and groan under all the reading and writing we're doing, especially with more user data coming in. Sharding seems like the logical next move, but man, it feels like a huge architectural shift. What's your take on database sharding? When did you know you absolutely *had* to do it, and what was the hardest part about getting it set up?