For all these things the device is perfect. Though actually browsing the web with a modern browser is a no-go it is simply too slow. Would give a lot for a modern replacement. I have looked into the Gemini PDA though it is not the same as you need to put that device down on a surface to really use it. Not so with the Nokia N900. If anybody knows of a modern equivalent, please tell me.
I specifically wish that device could somehow have modern production, modern software and support, even with the same screen. The design was not beautiful but by gosh was it functional in supporting beautiful things, like comfortably chatting with my future wife during university lectures.
The n810 was pretty though, and to this day the keyboard slide makes the kiddos smile like "whaaat" :-)
Then there was the stylus issue, where it seemed to be permanently incapable of reliable calibration. So much for mobile digital sketching.
I did some web dev work on the 810 while traveling, but quickly realized that my MSI Wind was like a speed demon in comparison. So it was more of a media player, clock, alarm, and ebook reader but it did those really well.
If it came back, it's the type of device I can see HaikuOS being a natural fit for...
There was a lot else good about that phone too - a full-fat gnu/linux device that actually fulfilled its basic requirements to function as a phone! Terminal as standard :)