In my experience, the Psion 5's are starting to break down now. I went through 4 in a year!
My solution was to buy a Psion 5MX, not because it's better built, but because it came out 2-3 years after the 5 and thus will last on average 2 years longer - which means they'll break in 2012. Also stockpiling the 5MX's so when mine breaks in 2012, I'll have 3 or 4 more to last me a few more years.
I've got a 5mx here that's not been used in a while, should have plenty of life left in it..... :-)
(Disclaimer - I stopped using it because I changed jobs and no longer needed its functionality, then eventually went for a netbook as I no longer so needed its portability and valued being able to run Visual Studio! I'd still recommend them to anyone who really wants a portable machine rather than just a toy though; in some ways I genuinely prefer it to my Android phone.)
I ought to add for those looking for a Revo: The inbuilt rechargeable AAA batteries are very hard to replace, and so Revo's (and Revo Plus's) on Ebay, so many years after being initially sold, are likely to have a poor recharge cycle. Stick with a 5MX as you can replace the batteries easily.
My solution was to buy a Psion 5MX, not because it's better built, but because it came out 2-3 years after the 5 and thus will last on average 2 years longer - which means they'll break in 2012. Also stockpiling the 5MX's so when mine breaks in 2012, I'll have 3 or 4 more to last me a few more years.