i have one and would definitely recommend it if you do not want to buy a extra set of pbt keycaps or want some fancy features like macro keys and do not use media keys.
backlighting can be done with diy or you can buy backlit onis from china.
however if you want to buy another keycap set i think you are better off with a filco or QFR. (also it is not as easy to mod the oni with something like a new plate because it has other mounting holes as the filcos/QFR)
i haven't found any quality issues yet. Somewhere i have a album from the oni pcb (will see if i can post it here later). It is up there with filco.