I haven't had one of the "Natural 4000 v1.0" keyboards fail yet. There was a run of them a few years back where the quality sucked though. I had new ones where brand new out of the box one had to mash some of the keys really hard to get them to register.
As someone else said though, they are very susceptible to crap working its way under the membrane since its oddly shaped (and there are a bunch of individual sections IIRC). I've fixed the coffee on the keyboard problem a couple times by opening them up, and very carefully cleaning them (both the contacts and the membrane) with isopropyl and lint free q-tips.
