Yeah I watched it.
Legion would struggle to even be a B grade character in the books. He was used as a minor plot device a few times. Interesting concept but not fleshed out fully, he could gain the powers of others in addition to being a telepath and was Professor X's son.
As far as the show goes, it's probably the best X-men related thing Fox has done with their license. Especially with various portions of the Fargo cast and crew.
However something about the soundtrack just put me off, I was constantly being bombarded with music which seemed unnecessary. Examples of effective use can be seen in Fargo and Watchmen the movie.
Also that mental health facility seems incredibly pristine and clean, as are all the sets.
They're only nags but hopefully the story gets a bit more fleshed out as the potential is there.