My second smartphone a, nokia 9210 from 2001 , with symbian OS series 80 , is still going strong too.
http://en.wikipedia.org/wiki/Nokia_9210_Communicator

However, after reading some messages involving components that get loose from the circuitboards, i think those older ones were better soldered than the new ones.

hii.. i need to ask what is the probability of n900 reaching it flash read/write cycle limits??
and how to prolong its life? ( like not writing to emmc )

i bought a used N900 and used it for about 9 months and then sold it to my friend and he is using it now for about more than 2 months. if you take good care of it you will not face any problem.

I have had my Nokia N900 for about a year, before it needed a hardware repair - admittedly, due to my own carelessness. It's repaired now, but still, three screws are destroyed, and I cannot risk taking it outdoors. Water-sand-dust-bullet proofness is quickly becoming one of my priorities, along with easy-to-take-apart-repair-and-assemble. There are plenty small scratches on the screen (I should not have put these doorkeys in the same pocket; it was only one time, though), and the speakers are filled with sand (windy weather, sandy beach, photographing, what can I say?). I am frequently afraid of dropping the device into water - especially since I have read how difficult it is to clean the screen afterwards.. And these screws are becoming the worst nightmare: they are soft, rusty, and likely need a special-size screwdriver. Ideally, the screws should be either multi-use, or easy-to-buy-in-any-local-shop.
In contrast, there is a dozen-years old cellphone, waterproof, sandproof, without a scratch on the screen, looking almost new. Admittedly, it has been wrapped in babbles most of the time, from the beginning, but still, it seems to be better suited to the dangers of everchanging environment.
Best wishes.

If you use it continually, i doubt you will get much more than four years out of it. Of course, if it sits on a shelf, there is no limit.

The charge/USB port is the most glaring weakness, I whince a little every time I plug into my reinforced USB port.
If you do the Palm Touchstone inductive charging mod you are free of damaging your USB port and that can be kept for USB host mode in the field and emergency charging.
http://talk.maemo.org/showthread.php?t=56950
The only downside is that the inductive charging requires a seperate charging logic board so your phone will act like it is on battery even when on the charging puck.

I was searching for Psion 3 and 5 series devices on internet because I have heard about them a lot and probably maybe saw one when I was a kid, and its amazing that a lot of devices from 90s are still functional and you can get your hands on them.
As its already said if you solder the usb port before it breaks, and dont drop N900 much i think it should be easily useable. On the software side its interesting to see that people keep on making new software for N900, Cant say how the web changes and how long the default microb keeps serving us.
I have dropped N900 a lot, I got it in december 2009, and just last month in August it fell from my computer table while it was hooked to the usb cable and now its microusb connector has problems, it didnt come out but it doesnt charge anymore unless I put some pressure on it while connected. I am planning to fix it myself, I am even planning to buy another n900 second hand but in good condition if I find it, I heard its available for very low prices.

I'm on my second n900 - the first one lasted 11 months before suffering the infamous "all telephony functions disabled" error.
I bought a used one to replace it, that one held out about 2 years before showing same error about a month ago. I applied ForeverYoung's fix and it's working fine so far. But I would definitely be wary of this error if you're planning to keep one for a while, especially if you're uncomfortable disassembling the device.

Its

just 4 years!! M already on 3rd year of use! N that too heavy use..
I
I love this phone!
And it has no such hardware problem. I just fear the read/write cycle limit!

Do you know how to flash or upgrade Nokia 7710 using Flasher Box?