I use my N900 a lot, but only slightly for actual phone calls, probably only 10 or 15 minutes a month average.

I use my hardware keyboard a lot mainly for Twitter and for sshing into my Linux servers. Also for VNCing into my mythtv server to schedule and manage recordings whilst the missus is watching soaps - I find the zoomability of PresenceVNC to be an absolute killer feature, and one
that I have not been able to replicate on my Ubuntu laptop.

Other than that it's just the general amount and all round beastiness of the hardware - stereo speakers and the 32gb onboard storage - coupled with a 32gb microSD, means I can a fair bit of my music about.

Managed to track down a spare LCD monitor at work with a composite in which works great the TV-out along with a fullsize keyboard and mouse when the N900 is "docked" - killer

The flexibility of the N900 is what keeps me with it. Only wish I could have gotton my hands on an n950.... maybe I should tweet Quim and see if there's any left!