My Sony 505 still works fine, but the battery is dying so I've switched over to using my Kindle 2 as my primary e-ink (e.g. outside/walking) reader both are just about 5 years old and work fine. I've even dropped each of them a couple of times (small dent in the 505).
I'd say they are about as reliable as most calculators. But I do have a TRS 80 Pocket Computer that I bought back in the 80's which is still working despite some liquid leaking flaws (not quite as bad as the one in the wiki pic

) in the display

.
http://en.wikipedia.org/wiki/TRS-80_Pocket_Computer I still use it to balance my checkbook.