Only have flown Ryanair once, and the flight was fine...I did come in with very low expectations, but found them to be no problem.
From what I have read, this is what I believe to be the case:

Price wise, there are a lot of nickle and dime type fees. But they are fairly clear about the fees, its not like you show up at the airport and they say "oh you want an actual seat rather than standing in the back, thats another 10 euros." The fees seem to be something that you can figure out in advance, so that you don't compare one airlines fare to Ryanair's fare, you compare the total package.

Operationally, it seems like 9 out of 10 times they are fine. The one (or maybe less) out of 10 that they screw up, though, they will screw you bad. Customer service does not exist. They don't care about your satisfaction or being what we might think is reasonable in the situation, and they will screw you in ways other airlines wouldn't even consider (delaying you two days, flying you to a different airport and saying too bad).

My synopsis would be that if you're flexible, than they're great. They're especially good if you want to take a trip that otherwise just wouldn't be in your price range. However, if you have children, absolutely must be in a certain place at a certain time, or have a complicated itinerary, you might want to look elsewhere.