At the last #webtuesday, Steve and I had the opportunity to talk about the 2 day Hackathon we held during the christmas break. (slides below)

If you want to make a hackday yourself be warned that:

• Setting up your environment (developer and production server) will take you much longer that you expect at the beginning. (took as about half a day)
• Keep in mind that working with new technology (especially if you're doing a hackday to learn something new) might be very cumbersome at the beginning. 
• Apart from the lack of knowledge you might have, there can be some delays trying to implement third party libraries (which might contain bugs themselves)
• To keep the team (or just yourself) focused we found it very useful to have a fixed deadline. Sometimes accountability might already be sufficient.
• Before you make your newly developed service public make sure you already have a hosting solution that allows you to scale if demand suddenly increases, furthermore be knowledgeable about where your data gets stored and who can access it. (remove all those "password" passwords at least)

At the end of the talk I demoed a short script to save tweets to a mongoDB database (you'll find the according source on github).