I tend to upgrade my computer as often as I can afford (and my conscience allows). With PDAs it was mostly dissatisfaction that made me sell them and get a different one, same with mobile phones.
E-ink devices have been different so far. My Iliad is a device that mostly does what it is supposed to do (apart from the battery life and slow boot issues) so I see no need to exchange it against anything. The STAReBOOK would be pretty cool if they had not stopped developing it's firmware (actually, I can't even get the old firmware form their website anymore which is not a good sign at all) and I guess the Bookeen device could be really great if they get the software right.
The only reason why I don't believe it's going to last more than those 3-5 years is that it will probably break or the formats that it supports will go out of fashion. It is also possible that somebody will come up with a device that serves as a Computer, Phone, Ereader and PDA all in one small package and without batteries (or with everlasting batteries), with different screen-sizes depending on need (maybe rollout display or something) and all those fancy things they keep promising us will be "coming soon"...but that's always a dream and so far it has not come true...
