I don’t have a DeMellier bag nor have I seen any irl, so can’t help there. But I have had a APC mini Genvieve for a couple of years and can share my experience with it.
Not sure which styles you are considering but I think APC’s most popular silhouettes are the classic small shoulder bag looks - the Grace, the Genvieve, the DemiLune. Imho, they are affordable dupes for the Celine Classic or Triomphe shoulder bag, especially the Grace.
Note these styles are quite stiff, clearly the leather is strengthened with a rigid layer, so there isn’t any give. I chose the Genvieve because of this - the accordion front and back sections of the Grace (like the Celine Classic) are pretty nonfunctional for holding anything thicker than paper. The leather is very sturdy - it will scratch but not easily, though once it is scratched there isn’t much you can do about it. It’s not of the same quality as a real box leather like Celine where minor scratches will buff out and develop a patina.
So far the construction of my APC has held up wonderfully - there are no popped stitches, no wear on edges, no discolouration on the hardware, or anything other than minor scratches on the leather that show a bit at certain angles in sunshine (nothing major). It’s nice and light, and I feel a classic style that can be dressed up or down. Sadly though, my new larger phone pretty much takes up all the space in the mini size so I haven’t been using it as much lately.
All in all, I think it’s a solid choice to add to a collection…just make sure you get the size you need and don’t pay retail! I got mine from Ssense during one of their twice-yearly sales for 40% off….at the sale price it was worth it!