I've tried Plex but keep going back to Monaco. Somehow, my eyes grow weary faster with Plex. This may just be a function of my age, so your mileage may vary, but here's a listing of things that I think make Plex tiresome for coding:
1. The lower-case ell ("l") and number one ("1") look so similar that I need almost to squint to see the difference, if I'm tired.
2. Plex seems to distinguish between letters by small "inside" features, as opposed to "edge" features. What I mean can be seen by contrasting "a" and "g" in Plex and Monaco.
I think it's this second thing that tires my eyes. While I admit that the letters are clearer in Plex than in Monaco (e.g. there's no mistaking "a" for "o" in Plex), I find that tired eyes can get enough information from the outside shape to distinguish the features, and there is a higher comfort level in viewing the characters, without the extra high-wavenumber twists and turns of the "pen".
Again, I think a lot of this may just be that I have old and never-especially-great eyes. But I thought I'd mention these things, in case younger folks find their eyes growing tired and are wondering why.