I've gone for seven. All I do on my reader is open a book, read that book and then go on to the next, I don't even turn wifi on as I bought my device to read books, not check email or whatever. Since I bought my Kobo I've had four updates, only the first came up with a bug that managed to arse me off, that was the cover bug.
Since that one bug, I've been more than happy, in fact seven now seems a bit low. Then again I haven't bothered updating to the latest firmware, judging by reports I won't.