Right. No one can say for a fact - other than Nokia, since only they know what they really expected and what actual sales and customer feedback were - whether the N900 was a failure. Everything we say here is just subjective and speculation.

Even so, let me make some points. To me the facts say that the N900 was not a failure by any reasonable measure.
It works well for most users' purposes.
It was in short supply for months so sales must have exceeded projections.
It showed that a real, full-stack root-enabled Linux distro-powered pocket computer could be let loose on the public with at least reasonable success.

If the N900 (along with the 770 and 800-810 before it) had showed Maemo to be a failure, there would be no MeeGo and Nokia would certainly not be betting the future of their high-end business on it. That they are seems to be a clear indication that Nokia was at the very least fairly happy with the results from the Maemo experiment.

So someone explain to me how the N900 failed...other than it didn't outsell Android or the iPhone.