We have Lazy Jacks on our 12.5ft long boom. There are two lines on each side; three would be much better and four would be ideal.
The Lazy Jacks are great when lowering the main, especially if you're "in a hurry".
They're a bit of a pain when raising the main; you have to be headed into the wind to make sure the battens don't foul the lines.
And, as Jim mentioned, it takes a bit more work to put the sail cover on.