Preliminary messing around with the little lens gives me good feelings.
When my FTZ II adapter shows up next week, I'll be able to test the 40mm f/2 against the AF-S 50mm 1.4G and 1.8G lenses (and maybe the AF-S 58mm f/1.4G, AF 50mm f/1.8D and 50mm f/1.2 Ai-S). The other Z mount lens I have is the impressive 24-70/4S and I'll compare its 40mm performance as well.
One thing I found after reading lots of reviews and tests and watching lots of on-line comparison videos: It seems that many Z-mount lenses like the 24-70/4S kit lens and the budget 40/2 non-S series lenses have optical performance a little bit behind their top-line Z mount relatives but seem to outperform even the better AF-S lenses of similar focal length in terms of sharpness; especially wide open and into the corners.
Watching Z vs. Z lens comparisons might make you not so enthusiastic about some of the lower-priced Z-mount offerings but seeing these lesser Z-mount lenses pitted against the good, better and best F-mount stuff can make you sit up and take notice. That's why I didn't hesitate to pick up the two Z-mount lenses I have.
This may or may not be a good thing, depending on your preferences. For example, I like the old 50mm f/1.2 Ai-S and the new 58mm f/1.4G because "they have character" rather than great MTF scores in lens tests. I don't care for the clinically sharp look a lot of modern lenses produce, although it is fun to have lenses that have incredible revolving power for some applications.
To each his own...