I have gone through five N900's over the years (bought my first one the day after they released it). The first one died due to the USB port falling out, but all the rest died because the modem-chip gave up. Once an N900 displays the "All telephony functions, including emergency calls, are disabled due to a communication error. To recover, you might have to reboot the device."-error message, you likely have lost your phone. I never was able to figure out what made it happen :-(

It still works with WLAN, and I still have one live at home as a small server and for the excellent "Evil Alarm" app that is the best thing to get me up in the morning :-).

I would say the Jolla is just as good for tinkering as the N900, and with much better hardware (though still just mediocre by modern standards). It seems clear that the Jolla is the best tinkerer's phone sold new today, but if it hadn't come out I would have bought another used N900 when my latest one died!