The only times I've had baggage problems have been when IB was involved. Every single time I've made a connection on IB, they've stuffed up my baggage. Either the system wouldn't let them check it through (on the same ticket to another IB or BA flight) leading to my having to collect and recheck at MAD. When they did a manual baggage check in BCN, it made the connection at MAD, then they never got it to BA at LHR and it was left behind.
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Getting way OT, but when has that stopped anyone ... Funny that you mention baggage. When you mentioned in earlier post about disinterested CSR, that would sum it up I think. Last month, we rather foolishly flew IB from BCN to MAD, it was 3300 Avios and about $35, whereas the train around the time we wanted to travel was 110 euro. Train would have been much quicker, as for some reason our baggage didn't make the flight (I think we got to the airport a bit late, we got there only an hour before departure, cutting it quite fine as baggage cutoff is 40 mins for domestic flights ;-). ).
At MAD we joined the carousel with about a dozen other people. About 30 mins after landing the 14 pieces of luggage came out. Ours was not in amongst them. So we waited until the carousel stopped and proceeded to customer service area. First - extremely disinterested agent said there were only 14 items of luggage checked to MAD on the flight, just go back to the carousel and wait 15 mins as they might send more luggage out. (Translation ... Hopefully by the time you get back someone else can serve you). So one of us waited whilst the other circled all carousels, checking the not insignificant amount of "stray " luggage littered throughout the airport.
Anyway, so we went back and had a different (equally disinterested) agent this time, who said,go back to carousel and wait 15mins, which we refused so she grudgingly filled in a lost luggage form, and said the system showed no indication where the luggage might be. By now the next flight from BCN was only 30 mins from landing, so we decided to wait, as we had a hunch it would be on that flight. That 30mins and the 30mins it took for the bags to reach the carousel. And sure enough there were our bags, so off we went 90mins later. But to sum it up my dealings with IB have been very ordinary (right down to the rather odd ushering forward of some passengers from Y to J upon reaching cruise, both times I have flown them between MAD and South America)