TL;DR: I really like it; seems like KeDJ material to be quite honest.
One of TMG's developer/designers is Seth Jaffee (of Eminent Domain fame in particular). In early 2014, I was at a game day with him and at the end of the night, he pulled out a prototype for Gold West. I don't know if TMG had officially signed the game at that point or not. Point is, I got to play it and enjoyed it.
Fast forward to one week past GenCon 2015. Although I wasn't able to attend GenCon, a friend of mine did. He was able to procure himself a copy of Gold West before they sold out of the limited quantity. I showed up to our local meetup and was ready to teach, having reacquainted myself with the rules earlier in the afternoon.
1. How has the game changed since I played the prototype? Adam McIver's work on the art and graphic design was obviously a huge bonus. The prototype was nice, though spartan. I don't feel that any of McIver's choices detract from gameplay. We played in some lighting that made the color palette slightly problematic (at a local pub), but I don't imagine this would be an issue in almost any other situation.
2. How has gameplay changed? It didn't feel different to me at all. The game had to be pretty much complete when I played the prototype, because nothing caught me off guard during the rules read nor the actual production play. I think the tweaks probably came in the Boomtown tiles and investment cards - I certainly don't recall what specific bonuses/investments we played with in 2014.
3. How is the rulebook? Really good. Great layout, clear definitions and instructions for the turn structure, Boomtown tiles and investment cards. I think the clarity and conciseness of the rules is akin to a game like Istanbul, which was very clear itself and most certainly helped that title win a major award (KeDJ) in Germany.
4. How do I like it? Obviously I am a fan (and not just because my name is in the rulebook as a playtester). I feel that the mancala-like supply track mechanism really works the grey matter quite well - not a brain burner, but certainly decisions to grapple with. The folks I taught it to only struggled with it for the first couple of turns before the lightbulb turned on. After that, their questions/comments were made in agony about the decisions they had to make, not about how the mechanisms worked.