Important, interesting, but dated.
Crisis on Infinite Earth's is a sort of major event for the superhero genre of comic books, the idea of a massive event that drew in every major player in the universe to fight some grand unifying threat. Though I think technically Marvel's 'Secret War' did it first, DC blew them out of the water here with the sense of both scale and importance. While Crisis Crossovers
have grown to become problematically common in modern comics, it was still exquisite for its time.
As for why the story is so interesting is because it served to reshape the DC universe into something more streamlined and easier to understand, back when such a thing was necessary because you couldn't just do a Wikipedia binge for information. it served as the swansong for the DC multiverse that had grown to become a rather convoluted mess containing so many universes and groups all existing at the same time, while also paving the way for a reboot that would undo the ridiculously overpowered nature of several characters.
The plot is... well kinda generic in all honesty. The evil Anti-Monitor comes essentially out of nowhere, putting every universe at risk by devouring them, forcing the heroes of several universes to band together against him. Despite the rather generic nature of the antagonist, there is still a real sense of scale to everything by having all these groups together, every major powerhouses both good and evil in such major battles. It does get a little clustered to have so many stories going on at once however.
George Perez' artwork is as gorgeous as ever, making for some stunning visuals and showing off a great silver age aesthetic. It's just a shame that same Silver Age vibe appears in other ways, namely the stunted dialogue that really showcases that this is indeed a book from the eighties.
Despite its aged and slightly cluttered nature, it's still an okay read and rather important in DC's history. Definitely worth a read however. New 52 sucks balls by comparison.