I have not seen the TV movie, but I have seen the Tim Curry movie, and the new movie, as well as read the book (about a third through my 2nd reading). There were a ton of things different in the new movie, I'll try to spoil as little as possible, and cover up what I can.
My biggest issue with the new movie is the way they portrayed some of the children. While everyone did fantastic in their role, whoever wrote them got some stuff seriously messed up. Bill was pretty spot on, I was just hoping that with the millions of time they mention it in the book he would have let out one iconic, stutter-free, shout of "hi-ho silver, away" instead of just having an easter egg in the movie (his bike had silver made into the side of it). In a way, it gave bill strength in the book, and it would have been nice to see them tie it into the movie. They made Beverly into this punk-ish style rebellious girl, when in the book she was extremely quiet and shy also, her dad was not a rapist in the book like they hint at before she is kidnapped. at least, not that I can remember . Eddie was pretty spot on, but he talked a lot more in the movie than the book, but honestly I think that gives him more character. He was kind of forgettable in the book. But, skipping now to my biggest gripe, They combined Ben and Mike's characters into one person, Ben, and came up with some completely different character for Mike. Mike was supposed to be the history nut that learns everything he can about the town of Derry and the strange presence there that makes children disappear because his dad tells him history about the town, and Mike has a general interest. Also, HIS PARENTS DID NOT DIE IN A FIRE. They are both alive and well in the first half of the book, and Mike his not some uneducated farmer. He does live on a farm, so they got that one particular thing right about Mike, but other than that, Mike is a stranger to me in this movie. Ben is not the new kid on the block, but his run-in with Henry Bowers was pretty accurate... at first. It was supposed to be a lot longer, and starts unraveling Henry's character. The only accurate thing about Ben was his obsession with Bev, and the poem he wrote. I could go on about the characters, but I don't want to overload Iyou's website lol.
I would, however, like to touch on the fears that the children see when dealing with It. The only one that's accurate is Eddie, but they don't give any background to it. I understand that they have to cut things out due to time, but without background on some of the things they see, you almost have to have read the book to know what's going on. Bev's fear was the 2nd closest, but it was not as extreme as the movie was. it was just a sink full of blood, with some splatters on the floor instead of an explosion over the entire bathroom but besides those two, nobody's initial run-ins with It are accurate at all. Especially Ben and Mike.
As far as Pennywise goes, visually Tim Curry is closer to the book. They actually compare him to Bozo in the book (good call, Iyou). The new movie fits the character better in every other aspect, though. It was supposed to be this being that hates children and lives off their fear. It was extremely creepy in the book, but at times could be a little silly, but just a little.
After my initial dislike of how far the new movie strayed from the book, I really started to like the movie, and I'm actually thinking about seeing It again in theaters.