I am a bit familiar, have been looking at the FP5 as a next phone, so will probably buy an FP6 assuming first batch of reviews aren't catastrophic.
Personally the appeal to me is the modular hardware support, but also the expected long term official software support (and inevitable community support when that ends). Also the principal that it's a company that is trying really hard to do things differently in a way that benefits everyone just seems sensible, innovative and something I want to support compared to other phone companies.
If your biggest issues with FP are removing the headphone jack, and requiring a small screwdriver to replace the batteries, then I'm genuinely interested to know who/what you're considering after as an improvement?
I love FairPhone's vision, and I think they do a pretty good job executing it. There's certainly room for improvement, but I've been really happy with my purchase. I also think removing the headphone jack was a direct betrayal of their ideals.
I've bought the FP3+ camera, and I'm on my third battery pack, degoogled with murena /e/-OS. Works perfectly, except the screen is a little too large for me. I wish it was a Sony Xperia ZX1 compact size, but one can't have it all.