I just can't believe it...

It's not bad but I don't think most people would describe it as a horror classic and a good one. I'm surprised you call it that having not seen the second part. While it's a mini series by definition I guess, most people just view it as a two part movie, that is basically what it is.

Yes, you are right, it may not be so good for some, but it is actually a classic, almost everyone has seen this movie or knows about it, and now I remember how ridiculous the final is

IT might be considered a classic and memorable due to all the hype and talk this movie got at it's release. I was 10 years old at the time and still remember the commercials and talk months in advance. For a made for TV movie on a major network it turned out fine, there was lots of limitations as to what you can fit in with the censors and limited time. It will be interesting to see how the new remake turns out.

The tv miniseries was/is memorable for one reason and one reason only...Tim Curry's excellent portrayal of Pennnywise. Period.