I have seen v6. Yes, it is tons better code. But it's stuck with a heavy legacy of zero design effort file formats which limit the power of the software in many areas and Infopop are unlikely to be able to get rid of any time soon. Something that really drove me off is their mess between wordlets and templates; their "templates" are a flat out lie since they're nothing other than a bunch of hardwired subroutine calls to generate the HTML and the wordlet system is a very inflexible, restricted way of modifying the looks of pages that depends on how much control Infopop or a hack's author deemed worth the effort to give you. Lastly, there is massive amounts of Javascript being thrown around, mostly as a quickhack to offload the "login" functionality to the browser rather than implementing it serverside.

For me personally there is also the fact that our board is full of custom hacks I wrote myself. I started porting them to UBB6, but the above factors eventually put me off before I could waste too much effort on it. Right now I'm hoping for a UBB importer to Ikonboard v3..