Everyone seems to be focusing on the +10% science and production. While helpful, this really pales in comparison to the early great scientist and engineer points. If you can keep your cities ecstatic and build 2 early campuses, that is +6 scientist points a turn. Classical era great scientists only cost 60 points so that alone is a great scientist every 10 turns! Add in the Oracle and Pingala's third promotion and it becomes almost absurd.
The problem with the GS's is the AI with bonuses will snatch the early ones really fast. You can rush campuses but if Seondeok is in the game, you're simply not getting it much less getting your cities to Ecstatic. With that mediocre start like mine in the above game where it takes like 20 turns to even improve a luxury, it's just not happening. The early GS's aren't very good anyways. Case in point if you build the Great Lib, you've already duplicated the value. It's a strong bonus, but there's so much that can go wrong.
I'm still going to stand by my point that GEs are the worst GPs in the game and the Mausoleum is also not very good.
I got every GE in the game, but the first wonder one didn't show up. Granted, I don't even think they're that great, because you've wasted a district slot and production in the hopes they'll show up. Maybe if I got harvest and can purchase it with faith and they're the first in the list, that might be worth it. As for the Mausoleum, you'd just be building a wonder to build other wonders.
So the first GE was the one that builds walls. Not terrible, but it's not like walls are expensive
2nd was the GE that increases district capacity and boosts printing. But I already boosted printing and it really had no use because all the relevant district is in place. I mean, sometimes food/amenities is low, but cities don't really need that many districts to begin with. This just basically lets you build an extra encampment or Holy Site.
3rd was the absolutely awful one that culture bombs when making an IZ. Of course I needed IZs in place to even get it, so it's a waste of space.
4th GE Workhops give 1 culture. Oh joy. Very useful especially when the game doesn't encourage IZs in every city to begin with and workshops are insanely overpriced to begin with.
Note that getting an extra charge to any of these would be pretty meaningless.
5th GE -- 315 to wonder construction. This is finally a good one. I use it to finish Ruhr Valley, but that's more of a sign of boredom than anything else.
6th GE -- 480 to wonder construction. I never actually get this one as the game ended, but I'm already out of wonders to really build.
Now, to be fair, I only had 1 IZ/pingala using the oracle. But otherwise I'd need to build a district that is terrible for half the game to get GEs that are terrible for half the game too.
James Watt: Assuming Mausoleum this should translate to +2 production in all of your cities. More importantly this gives you 2 free workshops and 2 free factories which helps you trigger the very difficult eureka for communism. When going for a sub 200 science victory it can often be difficult to get royal society early enough so I'll sometimes go the industrial zone stacking promotion for Magnus in my spaceport city which makes James Watt a must have.
Unfortunately, as I found out in my Germany playthrough, James Watt only applies the +2 production to the factories themselves, not the area effect. So unless you're putting a factory in every city and thus defeating the purpose of factories, it does not boost every city.
I think that drives home how bad they are.