As an n900 user for two years I have not touched it since I got my N9. The screen on the N9 is bigger and better IMHO. I do not miss the keyboard and as i have the 64Gb version the lack of sd card slot is not an issue either.

Have a search through this section teher are loads of posts comparing the two. They are different, the n900 feels like a shrunk laptop whereas the n9 feels more like a smart phone.

Things the n9 is missing vs N900:-
IR port (never had a use for this anyway)
Radio transmitter (again used it once i think)
Keyboard (swype on the N9 has made up for this)
Lack of Word/Excel editing solution
No flash in stock browser (but is available in firefox)

What the N9 has over the N900:
It is faster (especially starting the web browser)
Sat Nav that works (although routing could be better)
Media Sharing
Sportstracker
Better camera
Bigger screen
Brighter screen
Gorilla glass (although having to take gloves off to use it gets on my nerves)
Better Social networks integration
The swipe interface
Ability to use it single handedly.

These are all in my opinion. However I loved my n900 and could not see me stopping using it. But the acid test is that it has been untouched now for weeks. I use it to hold my work sim so it is useable but I just chose the N9 every time.