My thoughts (so far since I only got through season 3): This show started out fantastic in season 1. The plot was good, the casting was brillant (except for Iris), and the main villain is one of my favorite Arrowverse villains.
Season 2 was simply just good. Not great or anything, just good. It had an ok villain (Zoom), the casting was good and I loved that they had the original Flash, Jay Garrick. I love what an amazing impact on Arrow that Flash season 2 did mostly.
Season 3 was awful. The whole 'Flashpoint' storyline was lame and Iris becomes the main focus of the show. The best thing about season 3 was Savitar, and even he was awesome until a very point, the one where his identity was revealed.