The 1990 IT miniseries is primarily memorable due to Tim Curry's terrific performance as Pennywise, but he was quite disappointed by the ending. Realistically speaking, the IT miniseries has a lot of problems, mainly concerning the second half. It's natural that many look back on the whole thing with rose-colored nostalgia, but the fact of the matter is that Stephen King's IT book needed a better adaptation, which it got with the movies. Those weren't flawless either, but they were an overall improvement.
That's not intended to suggest that IT 1990 is bad, or not worthy of continued appreciation. It still has a lot of great qualities, even with its flaws considered. As mentioned, Curry's Pennywise is spell-binding, even if he was allowed to get a bit too silly later on. The child version of the Losers' Club is also excellent, and gives 2017's version a serious run for its money. It's just the miniseries isn't quite as great as some fans make it out to be.
One aspect of the IT miniseries that no one seemed to ever be truly happy with though, even back then, is how it ends. King's cosmic book ending is probably unfilmable as written, but the 1990 production definitely dropped the ball in crafting its own take. One person who very much agrees with that is Curry himself.
IT: Why Tim Curry Was Disappointed By The Miniseries' Ending
One of the most disappointing parts of IT's 1990 ending is its lack of Tim Curry's Pennywise. Instead, the adult Losers' Club encounters a giant spider creature. This is mostly true to the book, just simplified. While a giant IT spider has potential in theory, in practice, the creature's creation was rushed, and became a mix of a ridiculous looking puppet for close-ups and cheap stop-motion animation for movement. It looks awful, with the puppet appearing to have googly eyes. Curry hates the spider just as much as anyone it turns out.
During an interview with The Guardian, Curry said of the miniseries ending: "I was very disappointed by the ending, when I turned into a rather unconvincing spider." To really hammer his dislike home, Curry told Screen Geek the following, in a separate interview: "I hope they make the ending better because on [the TV version], I turned into some kind of giant spider and it was not very scary." Curry is, of course, 100 percent right. One wonders if the ending could've been saved by finding a way to incorporate Pennywise directly, allowing Curry to remain in the mix. Instead, the ending sucked the air out of IT's story.