I finished season 3 last night, after spending a month watching the series at an episode or two per night pace. It's the best sci-fi I've seen in a long time. If you're a forum regular in any of the sci fi threads (Star Trek/Orville, etc.) and haven't checked this one out, then do yourself a favor.
It took a few episodes to familiarize myself with the universe. I wasn't completely sold after the first couple of episodes, but the show sucked me in shortly afterward. I even started watching with subtitles, so I wouldn't miss anything critical. I agree with others that a strength of The Expanse is it's not holding your hand. This is fairly intelligent sci-fi that was marketed piss poorly by SyFy channel, so much so that I'd never heard of it. A couple of YouTube nerd channels pushed the show hard, and I finally made time for it.
I remember Shohreh Aghdashloo from "24" and was excited to see her here. She doesn't disappoint. I had just finished Chernobyl before beginning The Expanse and was not expecting to see Jared Harris again so soon. Two very different characters! I prefer Chad Coleman here much more than I do on The Orville.
Casting overall is solid. Favorites include Amos and Bobbie. Drummer came on strong in season 3. I enjoyed the dynamic between her and Ashford and how the story avoided predictability with their relationship. I also liked how Melba's arc resolved after not being wholly invested in her in the beginning. Season 3 finale was very good.
Thanks to book readers in the thread for consistently using spoilers. I haven't read them and appreciate not knowing what's coming.