I've never really been a big fan of Mechanist themed missions, but the Builder's Paradise is undoubtedly a huge improvement over Sabotage at Soulforge (which I personally found one of the most exhausting and unpleasant missions from the Thief 2 campaign). The massive amount of effort that was put into it genuinely amazes me: the brush work, voice acting, new enemies and writing are all top quality. That being said...

I have mixed feelings about the gameplay - specifically the keyhunt and the backtracking which are kinda annoying. Unfortunately, the layout is not only confusing, but the mission gives you only a somewhat sketchy map (at least still can be useful), and you gotta collect keys from everywhere. Not exactly something I find fun in a Thief mission. Also, while there are some smart loops you can open as you progress (thus you can't get too lost at the beginning), unfortunately the map is anything, but intuitive. There is a good chance you'll spend at least half an hour running around trying to find something - especially if you're going for completing the optional objective, which is a huge item hunt that feels like a justification for the enormous size of the map.

The Builder's Paradise is easily one of the most amazing (technical) milestones in the fan mission community, and a special experience that I think is worth to play at least once - but it's not necessarily a mission I wish to replay.