I never owned one, but I did have a couple of friends that let me use theirs. The touchscreen was a bit dodgy and the battery life was suspect - but IMO those are minor complaints compared to the main, insurmountable problem: the UI was extremely slow.
Like, tap-the-screne-wait-multiple-seconds slow. Even before the revelation of swipe gesture and smooth 60fps animations on iPhone, it was slow by the standards of its own era. I remember trying to use the phone to type out a quick text and being infuriated. Response time between keypress and UI update was atrocious, and even closing a window took well over a second, if not multiple seconds.
It may have been FOSS-friendly, but it wasn't really a good software stack otherwise.
Taking calls with the N900 was especially hilarious. The phone was ringing but it sometimes took many seconds for the phone app to appear on the screen. When you took the ringing phone from the pocket and pressed the green receiver icon to accept the call, it just before your finger hit the screen managed to rotate the screen and of course the red receiver decline icon was there in the other screen orientation.
That's some ingenious design easter egg and symptomatic of how unfinished this stuff was.
Also the Busybox default shell was next to useless. The charging microusb port also always broke. How many cents did they save per phone by using a substandard part? The keyboard was very good though.
N9 was so much better than N900. The graphics and the fonts looked so clean, and the swipe UI was just pure genius. You could see it had some serious thought put into it. Android UI is a mess compared to that. It's very slow to switch between apps and tasks in Android. If only N9 had not been so excruciatingly slow. It didn't help that multitasking was so easy.
I guess such incoherent products are a sign of a malfunctioning organization.
The N900 wasn't all that great as a phone. The OS was buggy and you had to go through hoops to do basic things like send MMS pictures. It wasn't the best phone, but it was an amazing pocket computer at the time. You could load a variety of operating systems and tinker around with it and it had a hardware keyboard.
Well it was not without its faults; the software was generally just buggy and laggy, battery life was quite poor, device was bulky and heavy, the resistive screen was bit iffy at times, there weren't that huge library of notable apps, etc.
And this is coming from someone with fond memories of N900.
But this is a matter of ideology.