I have to add to the last poster's description of FR.
I travelled with FR 3 times. The first two were bearable but the last one was a disaster which made me swear that I would never fly with them again and warn anyone who will listen to do likewise.

The best way that I can find to describe FR service is that it is like a hitch-hiking experience but instead of automobiles they use airplanes.
Just like with the automotive kind, most of the times it will get you (ultimately) to where you want to be. I would say about 90% of the times. However, just like with the typical hitch-hiking, sometimes you will not be able to get a ride or you will be dumped somewhere that you had no intention of being and then you will have to fend for yourself. No explanations or reasons will be given. No assistance will be offered.

My advice is this: before you decide to commit yourself to a 'ride' with Ryanair, ask yourself this: what will happen if my planned flight does not depart for a few hours or for a few days? What will happen if I am delivered to a different destination than the one where I wanted to be, sometimes several hundred kilometers away and maybe even in a different country? Do I have enough flexibility that I can work around it somehow? If you can then go ahead and take advantage of the sometimes low fares (as long as you don't check in too much luggage, can check on-line, etc. etc. etc.) This is especially true in winter when the probabilities of having some sort of adverse weather conditions that prevent FR planes from landing or taking off are much higher. Keep in mind, FR uses secondary, out-of-the-way airports because they are cheaper. Why are they cheaper? Because they do not offer the same level of service as the major airports. For example, very few of them have Category III landing systems that allow airplanes to land even if the visibility is minimal. Most only have Cat I or Cat II landing systems which require much better weather conditions to be useable. When you're talking about towns like Treviso or Hahn or Ciampino which are prone to fogging, you're asking for trouble.

If you know you travel plans long enough in advance, it's amazing how inexpensive air travel can be with airlines that actually care about their reputation, be they low fare or legacy carriers.