Although they are very interesting because of their prices, there are also some disadvantages you may know before choosing one of this airlines for your trip.
The low prices mean you will loose some of the services like the catering. The free newspapers are also out of their services. But the main disadvantage is the maximum dicrease of the aircraft infrastructure.
You must take into account that sometimes both the land and the take off are in secondary airports, like the Beauvais Tille in Paris if you fly with Ryanair, so you may add to the budget the price of getting to the city center.
You will have to pay for things like the overhead luggage and you will have to make your booking by the Internet.