United Colonies questline is very good. Every faction is a big hit so far. The only one I haven't gotten into is the cyberpunk megacorp one, and that's only because I'm trying to roleplay a morally-decent character and the megacorp is obviously shady as fuck. In terms of actual content though, it seemed great.
For anyone who's about to start the game tomorrow, trust me on this - explore New Atlantis just enough to get familiar with what it is and who the United Colonies are, do a couple of sidequests if any strike you as interesting (check out The Well), but then leave New Atlantis ASAP and go to Cowboy World to join the Rangers, or Cyberpunk World to join the megacorp. On the way to each planet you should find yourself awash in sidequests and random encounters in space, which are fun. Go back to New Atlantis only when you've gotten to grips with what the game is and the tone it's going for.
I don't know if it's specifically New Atlantis' fault, or just the fact that it's the first city in the game and new players will be confused by it as they don't understand the game's tone and design philosophy, but New Atlantis really does suck. Even now that I'm enjoying the game, I'm finding it a drag to go back to with its endless elevators and trams.
I think the issue is what darkpatriot mentioned above - it's deliberately sterile and boring, but new players won't realise it's by design and will instead assume, as I did, that the whole game is going to be dull. On top of that, the real meat of New Atlantis is in The Well, which new players could easily miss, walking around the endless sterile gardens and shopping centres instead.