So I finally got to watch Shadowhunters.
And okay: it doesn't follow the book (so far) that closely, but I can understand the changes necessary for a television show adaptation. Unlike a book, where we get sucked in by (typically) the writing style and character introductions first, a TV show is towards a different audience. So they need to give the general public more of what to expect later on - to be like, "Hey! It's worth watching if you stay with us!" type of thing.
That being said, I did thoroughly enjoy it. Anytime there is a book-to-movie/show or game-to-movie adaptation, I walk in with an opened mind - knowing that things are gonna change, but as long as they keep the main characters the same with similar plot points, I'm perfectly content (hence why I LOVE the first Silent Hill film, though it wasn't that close to the game). And I thought it was well done in the show.
Simon's reactions were hysterical and, if I do have one complaint, it's that he is WAY too good looking. However, that aside, his character hasn't really changed. I liked how Magnus and Valentine were given little snippets of their personalities and situations, as well. Even though we all know they weren't mentioned until much later in the book (again, they gotta hook the general audience). I liked how Iz made that comment to Alec, hinting on his crush - I thought that was cute and in-character for her.
My only complaint on Alec was that, for him, killing a demon in the book was a huge deal because he'd never really done it before and in this, he was slashing away with Jace and Izzy. I feel like that was a change that was unneeded because his shy demeanor (which is still there) was much more prevalent in the book.
I did like Clary's character - especially considering I could not STAND her until the third book. So that's a good note. I like her more down-to-earth character in this show and I hope it stays that way.
All that being said, I think it's a good adaptation and I look forward to seeing it every week.