I've been harvesting lettuce like crazy and it's been great. There's nothing like going out on my deck and cutting what I need for the day's salad. I was able to start cutting lettuce leaves about five weeks after I planted the seeds and it seems like the more I cut, the more they grow.
Once the weather gets really warm, the lettuce will "bolt", or go to seed and that pretty much marks the end of the growing season. However, because it's a cool weather crop, I'll be able to replant in the fall--probably around mid-September. So if you didn't plant any lettuce this Spring, I'll remind you that you'll have another chance when the weather cools.