I've used POSCA paint markers a little bit. I think they are fun; not sure how to compare them to Copic markers as I've never used Copic markers before, though I have used Prismacolor markers! I do know that both Copic markers and Prismacolor markers are completely different to POSCA paint markers in that the former are alcohol based while the latter is obviously paint based.
Blending with POSCA is hard to do since they dry somewhat quickly and can start to pill the surface (depending on the surface -- I've only used it on sketchbook paper. Oh and I also used it on fabric which was neat, though the paint bled a bit), and POSCA markers don't come in as many colors compared to Copic markers. The most annoying thing about POSCA markers though are the white markers -- I have two of them and they are both way too water-y! I can't seem to get them to a normal paint consistency no matter how much I press them down on some paper/shake them up/etc. But as far as the pros for POSCA markers go, they can be pretty opaque, matte, and vibrant in color, and since they are paint based they can be used on a bigger variety of surfaces compared to alcohol based markers.
I think they are worth trying if you have the money as they are fun to experiment with, but Copic markers might have better value if you are only looking to draw on paper.