i've never been a fan of the poly-800. actually, that's not entirely true because at one point (mid-late 90s) i saw it as being one of the few analogues i could afford. later on i owned 2.. a mkI that had really fidgty power issues and after i sold that a reverse keys version that i just had to have for nostalgic reasons. i sold that pretty quickly tho. it's not a bad synth at all and it has some nice qualities to it, but there are better options. it's alot like a jx3p really in terms of features and design, but without a better method of parameter editing (where the jx's and mk's had PG-XX controllers available the poly800 had nothing).
remember that old frustrating atari 2600 problem where the power input would always become loose and unstable? often requiring replacement or resoldering? that pretty much sums up my experience with the poly800.
other than that though, like many of the other posters... i find it really hard to pick something notoriously bad by Korg.. they have a pretty kick a*s track record. people early on when the kaoss pad came out, really liked to knock it down and talk trash.. but look how popular and feature driven they are now. i think that's one of the best things about korg.. they tend to pick up and improve their products over time based on their users wishes.. specially in modern times w/ the internet giving a better forum for feedback, demonstrations and knock offs. it kind of goes both ways tho because korg often tries to keep their product prices low by trimming down on something, like the sampling rate or build quality. they also do really quirky things that alot of korg lovers have come to actually appreciate.. like 4 voice synths w/ mono modes (mono/poly, poly6, ms2000, etc).
im so anxious for the DS-10 that i keep trying to forget about it. july is too far away and i dont like to wish for time to pass so its easier to forget about it.
about their build quality btw.. it kind of depends what you are buying. it'll either be metal and kick a*s, or it'll be light and plastic. but they're usually reliable in any case (outside of my poly800 experiences).