I’ve got a couple of litres or so of the Photocentric Grey Firm that I inherited from someone and I’ve never been happy with the results. It warps all over the place, lacks a lot of detail and always seems a bit tacky however much you wash and post-cure it (even under water). I’ve never been sure whether it was because of its age though. I’ve also tried their Flexible resin and that’s awful, it tears and crumbles if you do much more than look at it.
I must say though, that after an initial bad batch (confirmed by a very long and protracted (almost 4 months!) back and forth between me and PC doing test print after test print) I really like their UV Laser High Tensile Black. I’ve only printed a few things but in my experience it comes out nice and crisp and dimensionally as well as geometrically accurate. Although PC recommend using Black V4 in open mode I found that the prints came out much too soft with some parts of the print missing. I found by trial and error that the best settings for this HT Black resin are Black V2, again in open mode.
I hope you find this info useful.