Ryan Air

You bet there is a catch. Lot of fees. You can avoid them if you strictly keep with their instructions. Their airports are usually quite far from the city they advertise. For example: for Vienna it's Bratislava. For Prague Brno, for Milan Bergamo, etc.

They're extremely strict on luggage size and weight, they fine you if you don't check in online and print your boarding pass out at home before you get to the airport, and, as stated, they often fly to airports that are a considerable distance from the cities they claim to serve and sometimes have very limited onward public transportation options. (On the good side, the misleadingly named airports are sometimes genuinely useful for less obvious destinations - for instance Frankfurt-Hahn is close to some of the prettiest bits of the Mosel valley.)

There is an article in this weekend's New York Times Travel section on European budget airlines. It will tell you all you need to watch out for to avoid charges and surprises, and it is already on their website.

I can't tell you about hidden fees and restriction with RyanAir, but be sure you understand where the airports they use are. Of the 15 most popular airports on the western continent, RyanAir flys in and out of only three - Madrid, Palma, and Barcelona El Prat, although, in the case of Barcelona, a lot of flights use the more remote Girona airport. There are a lot of airports - Hahn (Frankfurt), Weeze (Düsseldorf), Memmingen (Munich West), Beauvais (Paris), Bergamo (Milan) - that are far away from the city RyanAir links with them, accessible only by bus, sometimes with no public transportation to them when a flight leaves early or arrives late, which Ryan flights often do. In my years on this site, I have worked with countless people who have blindly booked a RyanAir flight, thinking it would take them to a popular city, only to find they couldn't get to or from that airport at the time they needed to travel. I'm not saying don't fly RyanAir, just make sure you know where the airport is in relation to where you want to go and that you have thoroughly researched how you are going to get there.

I've flown a lot with Ryanair.
As everyone has said, you have to read the conditions carefully and follow the rules. There's NO lee way and you pay dearly for it if you mess up. Cabin baggage is measured and checked bags are weighed ruthlessly. The planes are bright yellow inside and you pay for everything on the plane (drinks, food). You can take your own food on board. Having said all that, you can get bargain flights. I have flown to Spain from the UK for £20. They have one of the best track records for getting in on time. The website tells you how to get into the city from the airport. I'll fly with them again.

I love Ryanair and am flying with them again next month. Whether or not the "remote" airports are a problem will depend on where you wish to go. For example, a few years ago I flew from London to northern Spain. The Ryanair flight landed in Santander - exactly where I wanted to be. And in places like Madrid and Dublin, they use the main airport. The "catches" are these: You must pay strict attention to weight/baggage limits. If you intend to check a bag, pay for that privilege while booking your ticket. If you want to risk carrying on and are refused at the last minute because the bag is too big, you will pay a lot more. Pay attention to check-in times. Their desk closes promptly and if you are late, you do not fly. Also, you must check-in online, but you can do that up to two weeks (I think - haven't checked recently) in advance of your flight. If you are willing to follow their rules, you can get amazing deals. My flight next month from Glasgow to Barcelona (main airport, not Girona) was 38 GBP, taxes included.