I've bought 7 e-Ink devices in the last few months to decide which one is right for me, only to decide none is perfect.
Nook - I'm keeping it mainly because I like the ergonomics, B&N selection, and I like the idea of having a device sold by a "big guy" that is likely to stay in the ebook business, and there are lots of third party accessories available. Battery life and lack of organization are disappointing, but not critical. I'm using it mostly for impulse purchases and library books, which make up the bulk of my reading.
Opus - I love how portable and lightweight it is, and it's support of folders makings organization easier for my purchased series books. Drawbacks are that it's a bit buggy and there is almost no third party accessories. I'm trying to find a good clip light for night reading, with little success.
Sony Pocket - packed up and ready to go on eBay, but I keep pulling it out and fondling it. It's metal construction feels more sturdy than the Opus, the light cover is fantastic, and the Collections feature in some ways is easier than the Opus folders. But I'm pretty sure Sony is circling the ebook drain, and there isn't room to hold enough books for me.
But I really can't justify three readers. I've created a nice distinction between having two - purchased books vs. borrowed books and have set up my Calibre library to reflect that. Adding a third to the mix makes it too complicated to keep track of what book I'm reading where.