Avoid: Iberia. I flew them for the first time in May, twice, from Madrid to Lisbon and then from Porto to Madrid just over a week later. Never again, if I can help it.
Both flights were basically on time. No complaints there. But I was hassled on both flights to gate check hand luggage, on Airbus A319s which are not tiny. My 22" carry-on fits vertically in the overhead bin. I'm not used to being hassled like this by European airlines: Air France, KLM, Croatia Airlines, FinnAir, airBaltic, etc. have let me carry this bag on their planes, even when full. Even on prop planes I've been able to leave a larger bag on a cart when boarding the plane and retrieve it immediately as I deplane, but usually I can just carry it on. Not Iberia.
On the first flight, my final destination was Lisbon so checking a bag wasn't a big hassle, though it cost me a half hour at the airport waiting for it. I had had cheaper choices to get to Lisbon e.g. TAP which had an option to let me carry on the one bag in addition to the personal item. That flight left an hour later from Madrid, but I lost a half hour with Iberia because I had to wait for the bag at baggage claim. And I paid about $25 more.
On the flight back, only a few of us were hassled and asked to see if our bags would fit in the "cage." The guy in front of me had to check his bag - he had a US passport so I assume like me he was connecting to a flight home. I had only two hours to connect in Madrid and didn't want to have to worry about exiting security in Madrid to go claim my bag then return through security again. (Someone else here said Ibera did this to them but checked the bag through to the US, even with just a "hand tag" (not computerized and tracked) and their bag made it on time, but that would have given me a lot of anxiety.)
As it was, my bag didn't fit in the "cage" either but when I removed something out of it the agent let me go with it, even though it still didn't quite fit. I had a different agent than the guy in front of me - perhaps why I got through.
And then there are the seats: the coach seat on the first flight was the most cramped I had ever sat in! My knees were jammed into the back of the seat in front of me - and that was before the guy sitting there decided to try to lean his seat back! Sorry, not going to happen buddy. I've flown Southwest in the US many times, and their seats seem like first class by comparison.
On the way back, my Iberia flight was part of an AA "award" flight, and somehow I got assigned an "economy comfort" coach seat in the front. I assume it was a mistake, but they didn't tell me that. Instead, they told me at the gate that someone else was sitting in my seat - ??? Really? So they made me wait to board at the bottom of the jetway until someone could come out and tell me my seat was in fact "available." No explanation was given, but when I sat down and realized how much leg room it had compared to the one on the first flight, I deduced that they had been trying to sell my seat as an upgrade and only let me sit there after no one paid for it.
Although both flights were short, there was no drink service at all (I'm told nothing is free in coach, anyway). I'm glad I had refilled my water bottle! But the flight attendants had time to pass through the cabin trying to sell duty free crap.
I thought Iberia might be a step above budget airlines because they are a "one world" partner (with AA etc.) but not so - RyanAir couldn't be worse than this! How foolish I felt paying more for Iberia than TAP just to be treated this way. Never again, if I can help it!