I don't know about plastic logic but I do know a bit about irex
I own an illiad and I'm sorry to say its software is complete and utter crap. As is the user interface. The basic things are really only basic and are not at all written with overcoming the limitations of the hardware in mind.
For example, rendering a PDF page takes a good few seconds. So I skip to the next page, and I have to wait. I skip back, I have to wait again. Well more than the delay the screen imposes. Despite the thing having enough memory to cache the last few rendered pages, it just doesn't. Neither does it pre-render in some low-powered fashion the next page while I'm reading this page, and yet its battery life is measured in hours, not page-turns.
There are a number of other things like that where they dropped the ball, including the hardware. Like how the screen developed a dead line three weeks after I'd bought it (second-hand for a third of list or so, still too much), by which time they'd already gone bust. There still were support sites up, but after they'd made you register and login, they took your complaint then just pointed to other sites and never ever answered. Now it's not entirely surpising they weren't providing support after going bust, but they didn't clean up after themselves either. They raised expectations and didn't live up to them. And apparently when they weren't bust yet they had to replace a lot of broken displays under warranty, as (I found out too late) it is a common problem. The entire thing has a fairly amateurish smell to it, for an unjustifyably high price tag.
Maybe they thought that branding the thing "enterprise" would be enough to gloss over the deficiencies, and they bet pretty heavily on electronic delivery deals as they did with a national "quality" (and struggling) broadsheet. Not the most solid of moves, judging by the results.
As I said I don't know about plastic logic except that I really didn't see their devices next to the various other e-ink e-reader brands that you also don't see much of these days. It's a pity, really, because I fully agree the technology is neat and probably a good fit, but, well, it's now thoroughly out-shined by tablets. So much more's the pity.