The Cons & Challenges
There isn't much for me to nitpick here. My only real problem was that this lens could only pull focus at around 2 feet, which feels much more expansive when you use this further in the field. There would be times when I wanted to push in for a tighter frame, and it just wasn't possible, whereas other lenses that share this 40mm focal length have trained you to be able to get into a pocket to snap your shots.
It may be a dealbreaker for some, but given the vast amount of work you'll be doing with this lens on your GFX camera, you probably won't hit this roadblock often. Besides, there's something else that's far more egregious: the lens hood.
This lens hood is ugly and barely works. It somehow makes this industrial, cold, ominous product seem dinky. In any case, there isn't much else to complain about this lens. It's excellent and brings so much value to the table.
The Competition
Compared to the market, this lens sits between the GF 45mm F2.8 and the GF 63mm F2.8, lenses with a 36mm and 50mm equivalent focal length, respectively. If you prefer one of these focal lengths over the other, this isn't a tough conversation; you pick the one you like. That said, if you're willing to be a bit more open, this is an interesting comparison to have.
The GF45mm is noticeably sharper and brighter; the GF63mm is probably just as strong and has an aperture of F2.8, so on paper, they seem like the better product. However, they're both considerably larger, and neither has a linear motor to focus as quickly as the GF 50mm F3.5. So, if you're a GFX travel or street photographer and looking at which of these lenses to get, you have this dilemma between the ceiling of image quality and operability in the field.
I've spent considerable time with the 45 and 63 and went with the GF 50mm. For me, I was happy to forgo the marginal gain in sharpness and light-gathering for a lens that would make me shoot more. With a lens like the GF 50mm, your entire camera kit feels more approachable, where the idea of having it by your side for 6-7 hours doesn't seem like a death sentence. Now, I know this will vary from person to person, so I can only go with my experience, but if you're open to any of these three focal lengths being part of your GFX solution, I genuinely believe the GF 50mm F3.5 brings the most value to the table.
Final Thoughts
The GF 50mm F3.5 is not a lens you buy for the aperture. You get this lens because it changes how you work and, more importantly, how often. The small profile, the fast focusing, and the premium construction make this an empowering lens to have in your kit. If you are already or hoping to bring your Fujifilm medium format camera outdoors more often, I believe this lens must be on your shortlist.