If you have never read the books, the show is good. I like the casting, the visuals, the pacing. However, it deviates so much from the books that it is disappointing. I don’t expect TV/movie renditions to be exactly like the books they represent, but I do expect there to be some similarities other than the names of the characters and the general world they live in. For example, (spoiler for season 1) there’s not an alternate universe and Valentine isn’t really discovered to be behind everything until Clary finds him (with Jace) in the old mental hospital (not Chernobyl).
If you can look past the major story line deviations, you will enjoy watching the series.