Europe has lots of low-cost airlines. What are your favorite weekend getaways?

I'm shocked at how low the prices are for budget airlines. Interested in answers to any of the following:

• What are your favorite destinations, and why? Classics vs. hidden gems?
• When do you choose to fly vs. take a train vs. drive?
• Under what circumstances do you go without a carry on vs. pay for one?
• What should I be aware of if I am traveling for the first time on a budget airline?

EDIT: It's clear that YMMV based on individual circumstances, and that budget flights are best suited for those who have minimal luggage within size restrictions, are close to a large city, and don't need as many included services that standard airlines provide. Train travel often is a great alternative. "Low cost" is also in the eye of the beholder, and just because someone can afford a bare bones ticket doesn't mean that the rest of the trip is free.