I used a N95 8gb for two year and a N900 for two week, I must say that the N95 is way better for me than the N900 for the moment (I hope it will change with the updates).

For the OS part :
- No portrait mode for the N900 for the moment : you need two hands to use your phone, not very convenient in the metro, bus, street...
- I don't like so much to launch a phone appli to make a phone call
- With my N95 I could sync mulitple calendars and contact via html syncML with google and goosync. No html syncML for the N900 for the moment.. This is VERY annoying for me!
- Control the music with the handsfree kit. No control with the N900 : in order to stop the music, you have to take it from your pocket, unlock it, find the player in the open apps and stop the song...
- impossible to phone directly from a SMS view, you have to go back to contacts, and, if the number is unknown, well... note it on a paper, 'cause there's no copy/paste here!
- no #.......# codes
- I miss a bit the MMS but not so much
- cannot easily change the volume while browsing
- battery doesn't last long...
- each time you connect to the internet, the N900 keep connected : on have to manually unconnect it. I guess it uses a lot of battery.
- no icon for new voice message : I had 13 new message when I had the idea of calling my voice mailbox
- ok, browsing on the web is 1000x better on the N900 than on the N95, but web browsing on a phone is just for occasional use for me. I don't want to spend hour on the web on a phone. But still, it's a very nice browsing experience and it can be very usefull in many cases.
- ok the larger screen of the N900 is wonderfull for watching movies in the train. The screen of the N95 is way too small. But you can hardly watch a whole movie with the N900 battery...
- the photo part seems similar on both phones, maybe a bit better (faster!) on the N900.
- no radio for the moment on N900. I use it quite often on the N95.

I also miss some apps, and hope they'll come to the N900 :
- Sygic drive for the GPS (I know it will come soon! I'm waiting for it)
- Google maps...
- a better Nokia Maps for offline maps
- Joikuspot
- no gmail app (I don't want the push mail), I have to go to gmail mobile website, not so nice.

The N900 is a very promising phone, I like it a lot, and I have no doubts it will become soon better than the N95 and than any phone, but for the moment it is still very new. If syncML via html doesn't come soon, or another solution (with MfE?), I will consider selling my N900, cause I really need a solution for mulitple calendar sync with google calendar...