I've had my Sony prs 505 since the end of 2008; I've used it almost every day, and it's still going strong (touch wood!). It's had a battery replacement during that time and does occasionally do a 'restart' on me, but on the whole I've been happy with it.
However, I've heard that Sony's standards have slipped since the days of the 505 and I can't vouch for their latest ereaders. If I ever need to purchase a new ereader, Sony's will be the first range I'll investigate. Kindle will be the last.