They're pretty much just Christian airport novels which you don't go into expecting high quality from. That said, they're an absolute cultural landmark you'll never hear the secular media referring to. Fucking EVERYONE in the Evangelical community had read at least the first book back in the 90s and 00s, and anyone who hadn't read the version aimed at kids. My family read these books, all their friends involved with a church did, they'd talk about it and say how it brings the Book of Revelation to life and how creepy it gets, that sort of stuff.
I'd almost compare it to the Harry Potter phenomenon of that era, except obviously a lot people into Left Behind thought Harry Potter was demonic. The Rapture doctrine as portrayed in Left Behind is 150 years old, but a lot of people who believe in it ended up thinking it would happen like in the books. They'd get raptured and get front row seats from Heaven to watch some Eastern European weirdo (or the Pope/US president/Bill Gates) who is secretly the Antichrist using the UN to take over the world via WW3 while all the shit from Revelation happens. Lots of obligatory Israel worship and Zionism of course. You also had stories of people who'd warn people about flying on a plane since they got afraid the pilots would both get raptured (or the myth they don't let two Christian pilots fly together so that doesn't happen). Or what happened when I was a kid where a kid down the road who went to the same church and school as me knocked on the door one day and was afraid the rapture had happened because his parents weren't home and forgot to leave a note. It's a hugely influential book on that culture, and anyone interested should definitely read it.
I only read the first two books when I was like 10-11. They're really silly at times, like the Antichrist guy isn't ever portrayed as eloquent or Hitler-esque, he just had magical ability to make mundane speeches and dialogue warp your mind unless you pray to God first. I didn't get much past the nuclear war in book 3, it just wasn't very entertaining at that point. I think it was so big for the same reason all the big Christian rock bands in the 90s were--it was an adequate copy of secular media with slick style that affirmed the beliefs of its audience, and was done better than other Christian knockoffs of secular media.
I only ever saw the first movie. I think it had Kirk Cameron in it since he's in every Christian movie. It was kinda dumb, but it's absolute god-tier compared to some of other rapture movies out there I remember seeing when I was a lot younger since it actually had a budget. Never saw the Nicholas Cage version, I heard that one was controversial because it doesn't follow the book, doesn't have enough Bible stuff in it, and has Nicholas Cage in it instead of a good Christian star like Kirk Cameron.