I quite like them. Most tiles are worth working in the endgame, so I'm often looking at total yields rather than individual tiles. Forest + lumbermill is about on par with a mine (no chance for discoveries, but a little health).
Replacing mines with flatland lumbermills means I'll get the opportunity to build more windmills at the expense of farms or cottages. I like windmills, considering them the best improvement in the game with all benefits and still rather good without Environmentalism.
Flatland lumbermill and 2 windmills vs. farm and 2 mines: You lose a hammer but gain between 2 and 8 commerce.
Replacable Parts can be a defining economy tech, imo a hippie economy is a viable alternative to farm/cottage/workshop spam. One drawback is that you'll have to make do with 'filler' tiles in the early game - forests can work, collossus-boosted coast is better.