I've only had a minor delay when flying from East Midlands to Jersey last year with BMI Baby, which was due to the plane arriving late from it's previous flight, although when it came in it was empty, and also a 737-300 rarther than the booked 737-500, so I expect it was infact a replacement. It was also delayed on it's next flight from Jersey to Manchester. That is a problem with the low cost airlines, since one way they keep costs down is by having quick turn arounds (usually just enought to get passengers and bags off, quick clean and check, and get it loaded for the next trip, oviously fueling whilst this is going on) so any delays tend to have knock on effect with later flights, infact I expect the plane was running late all day. We were only about 10mins late though, and being quite lightly loaded boarding was very quick (barely found our seats and sat down and we were pushing back). The weather was quite bad though, so quite a lot of flights were delayed. We expected delays or worse coming back though, since the weather was awful, especially the night before we came back, with strong wind and bad visability, and the newspaper flight from the mainland was about 5 hours late, but it cleared up before we left the hotel, other than a downpour whilst we were checking in, and we were bang on time. Landing at East Midlands was rough though (got a video of it in my youtube link). Still, good overall experiance and I'd use them again, especially since unlike most low cost airlines, they allow you to prebook seats (17F, just behind the wing). Definitly beat the alternative of long train journey followed by a few hours on a Ferry (particularly since a lot of ferries were cancelled due to the weather).