I really liked Deadwood first time I watched it, but found it nearly unwatchable when I returned for a second look around 2013.
Few glaring problems: Bullock and Starr are way too boring to hang a show off, as is Alma Garrett for that matter, Trixie's whore with a heart of gold is wildly unconvincing, and the way Swearengen becomes more or less one of the good guys by the third season is just a contradiction in terms, because he's a repellent, child murdering arsehole.
I will go into bat for the supporting cast, though, Robin Weigert, Brad Dourif, Dayton Callie, Ray McKinnon, Garret Dillahunt are all great.