Browsing - yes, right now Maemo seems to have the best browser.

VOIP - I couldn't say.

Contact Management - no, Maemo stinks at PDA functions and third party apps are not nearly as advanced as three year old apps I was running on my Palm. This, of course, could change but it never did for the previous version of Maemo on the N800/N810.

Upgradability - unknown, Nokia did not provide an upgrade path for Maemo 4 to Maemo 5. They have not made any official announcement that Maemo 6 will be compatible with the N900.

Stability - no, the current version of Maemo on the N900 seems to have more issues than the competition. A new version of the firmware was originally planned for Christmas but it has not been released yet.

Conclusion - I'd at least wait and see what the next firmware release brings. That's what I'm doing now. I'd also look at Android. It's Linux based as well. Beyond the app store, you can get root and people have done many creative things. Plus, at the moment, there seems to be a better phone underneath the enhanced features.