In the past year I have flown with Ryanair around 20 times and BA once. The BA flight was to New York from Heathrow. This is definately not becasue I prefer Ryanair to BA. I would choose BA anytime but its simply convenience. When you can fly to Rome for £25 return with Ryanair from Stansted, and thats including all fees, taxes and charges then why would you pay well in excess of £100 to fly BA from Heathrow. That is only based on where I live (around 20 mins from Stansted so Heathrow is usually out of the question anyway for a short haul flight). The airport Ciampino that Ryanair fly to in Rome is actually much closer than then main airport that BA fly to. When ever I do fly BA I love the service you get, I think its just exciting to fly with them no matter what the circumstances are.
But when Ryanair offer flights so cheaply you normally don't mind the level of service you get. The only bad thing with Ryanair I have found to be is no seat numbers. In my view that is one of the only things dragging them down. If they simply had seat numbers then I generally think people would have a higher image of them.
I go the same place in the Costa Brava several times a year. I can either choose Ryanair from Stansted - Gerona for around £30 (the cheapest has ben £16 inc all taxes and charges) and then a 20 car trip. Or BA from Heathrow or Gatwick for around £100 and then over an hour car trip to get to Heathrow and then another hour once I get to Barcelona.
Finally, after my little rant, when Ryanair can fly you from London to the other end of Europe for less than a trian ticket into London from Essex then why would anyone not fly with them.