I've luckily only had to complain once - got a phone call at lunchtime for a flight departing at 20:00 from BHX, saying there'd been an equipment change, and would I mind departing ex LHR instead.
Thinking they might give me an F-op-up for my trouble (I was booked in J) I said I'd do my bit to help them.
Worth pointing out that I only live about 45 mins from BHX though, so this was a bit of an inconvenience.
They said they'd send a driver straight away. Got to LHR and they had no idea what was going on, and at one point told me I'd have to pay as this was a change of flight
Didn't get upgraded, but couldn't really complain about that. What did upset me was that I was only credited miles from LHR and not BHX (admittedly not a huge amount, but still)
Sent an email summarising the whole thing. Got a response within a few days saying they'd get back to me, and it could take 30 days. Exactly 30 days later I had an email with an apology and 25k miles.
I'd actually only asked for the difference!