For better or for worse, it's going to be successful because Riot knows how to appeal to a mass market, not to mention the amount of people that are already hooked into League.
I haven't played these mainstream casual games in a while. I tend to stick to the competitive alternatives. DotA > LoL . CS:GO > Literally everything else.
Well it's still early in development so I can't judge much. I like some of the ideas they have, and I know they will improve it obviously. The graphics need a lot of work still, but one thing that turns me off is the theme mismatching of the characters and the environment. The characters have a futuristic theme while the map has fantasy theme. I'd like it more if the maps look more like the ones in overwatch. Other than that I like that they are going to spread a lot of servers around the world so we will not have ping issues, which is really important in fps games.
I personally hate how it looks. The concept just seems off to me. It just looks like csgo with abilities that don't belong in a csgo type game. I feel like upon release, it'll evolve into a "this champion must be picked or we lose" game, given how different each character seems to be.