I would say my COOL-ER is "good enough" but I wouldn't say I'm entirely happy with it.
There are entirely too many quirks - e.g. occasional reboots when I plug it into my MacBook, sometimes the list of files refreshes when you insert an SD card, sometimes it doesn't and you have to restart the device to see the SD card's contents. I also don't like the fact that one can't change the margins and that font sizes are enforced by the ebook itself (most ebooks I have seem to go from minuscule text to "157pt Sun Headline" size) - thus requiring me to faff about in Calibre and play with font size to get a comfortable (for me) reading experience. The interface, when moving from book to book, is appallingly clunky (and I'd love to try other firmwares - they can't be any worse, surely).
All that said, though, once I'm in a book and reading it works reasonably well - hence the "good enough" comment - ultimately at the end of the day all you want it to do is display pages of text.
TL;DR: I wish I'd bought the smaller Sony for a few quid more. I was swayed by a bigger screen for less money. But at the end of the day it displays books well enough. It'll do until a killer device comes onto the market.