I watched the first half of the pilot on Tuesday and here are my initial reactions:
It is good to have a space opera fix - and The Expanse feels like space opera even though the science fiction is very hard. The space battle at the end was staged with some awareness of physics and ballistics, shooting at ranges of kilometers and torpedoes taking time to reach their target. (I'm unable to find out anything about the backgrounds of either of the collaborators who write together as James S.A. Corey, but I wonder if somebody was in the navy?).
The plot is intriguing and the visuals do remind me of Babylon 5 - my big hope for this show was that it would bear some resemblance to B5. I liked the asteroid CGI and the ships, although they didn't give me the YES, that's IT reaction I had to the Hermes in The Martian, looked pretty good.
However, I had some problems:
1. Everything was too clean. We're told in the introduction that water and air are at a premium in the belt. So, why does everyone look like they showered an hour ago? Some of the characters had extravagant hairstyles. Part of this is the usual issue with television budget CGI (B5 had some of the same problem), but you could grime up your actors a little when they're supposed to be ice miners.
2. There was a zero-G sex scene which I swear looked like the "bottom" partner was lying on a standard gym weight bench. Come on, SyFy, you can do better than that.
3. The dialogue was a little rocky in places.
4. The most interesting characters all die at the end of the first episode.
Still going to give it a good chance, though. There are definitely some very interesting concepts here. (I haven't read the books...yet).