If you can find this at a decent price - I got my set for just over £5 - snap it up for nostalgia value and sheer entertainment.
The series starts off very stylishly with the soapy, melodramatic Airport (1970), and declines in quality over time. My favourite things about the original are Alfred Newman's classy score, the schmaltzy gimmicks (such as the overused split screen) and the excellent cast.
Airport '75 is surprisingly well made, with spectacular photography and generally high production values. At the same time it is terribly cheesy and replete with bad dialogue, making this the prime target for the later Airplane! spoof. But it's fun and occasionally suspenseful. I enjoyed cameos from the likes of Hollywood old-timers Myrna Loy and Gloria Swanson.
Airport '77 was always my favourite as a kid. The production values are poorer here (the jumbo jet bobbing its head out of the Atlantic waters looks like a cardboard cut-out), and there are patches of dullness, but again it's generally entertaining.
The Concorde: Airport '79 is BY FAR the worst in the series, and it is no surprise it was laughed out of the theatres in previews. The script is painfully bad, and the plot is outright ridiculous. The height of silliness is when pilot George Kennedy diverts a missile by opening the cockpit window of a concorde and firing a flare gun *while flying upside down.* You can't make this stuff up. Well, evidently someone did, and probably even got paid for it. I felt I should have been paid to *watch* it.
Nevertheless, their God-awfulness is a major reason for watching these movies. I enjoyed the set, and if you grew up with the series, you undoubtedly will too.