Well, I just read on various forums that Iberia is notorious for losing luggage. I land at night (not a direct flight, ORY-MAD-EZE) and the next day I have a flight from AEP to FTE, so I don't want to chance it. I don't plan to bring so much stuff as I plan to do lots of trekking and hence carrying most of my stuff on my back. I figured if I wear all my bulky clothes, my backpack will be half empty and then for sure they will let me bring it on board. That's not the problem, I have seen other people bring in mountains on board!
Another issue is the swiss army knife, which I will never see again if I check it in separately (been there, it's my third knife!). My boyfriend once forgot to check it in and they let him give it to the cabin crew to hold for him for the duration of the flight. Sometimes, they scan things but let things slide, depending on the airport, and the person doing the checking. I can recall times when I 'smuggled' such dangerous things aboard as: tweezers, fork, lighter...
I suppose it is just a russian roulette either way. Flying today is just such a nightmare- expensive, huge ordeal, under inhumane conditions with those tiny seats, but no choice :-(