Here's my take based on the n950 I have:

Even with the Beta1 firmware, the n950 was my primary phone within 10 minutes of use. It just worked perfectly as a phone.

email: imap, pop, exchange all worked with minimal and extremely simple setup. Sync happens smoothly, does not interrupt the bluetooth connection or phone conversation. This was one of my major peeves with the n900.
Bluetooth connection to my Jabra halo and the panasonic headset worked perfectly.
Tethering a laptop across bt was quick and simple enough for even an inexperienced user to easily manage.

Browsing: the default browser was fast but did not support flash. it crashed on ssl pages sometimes. With Beta2, all crashes were gone, though some pages still couldn't be rendered. Curiously enough, these pages didn't render well in desktop chrome either. I didn't bother to investigate further.

Software keyboard: Wow. So much better. Within a day I could see why Nokia decided that for most normal cases the N9 doesn't need a physical keyboard. It's such a pleasant change from the n900.
On the n900, my default action to do *anything* on the device was to slide out the keyboard.
On the n950, I've often forgotten that the thing actually has a keyboard.
I realized this when I switched back to the n900 (for development) and noticed myself always opening the hardware keyboard rather than use the software one.
Also: auto-correction is present and is less irritating than the n900. On the n900, if I miss-spell something, the software input panel (sip) would auto-magically remember that misspelled word. Eventually bad spellings crept into the dictionary and that was frustrating.
In the n950, the user needs to actually click the miss-spelled word and chose to add it to the dictionary - just like any decent word processing software. So much better that way.

SIP, skype and jabber (gtalk/facebook) are built in. Facebook and Twitter clients are built in.
From what I've heard, MMS is built in. I've not bothered to try it out - no one I know uses MMS.

All apps are much more responsive. My Qt apps load much faster.

The inbuilt camera app is good for quick snaps and videos, and also has enough knobs to take really professional looking photos.

Battery life: Consistent skype traffic for about 4 hours brought the battery to about 20%
On the other side of the spectrum, I've managed to forget to charge the n950 for about 2 days and it was still alive.

Sorely missed: macuco2, firefox, opera and so on. I want more browsers dammit! But thats probably just me.
Also missed: openvpn, vnc client, rdp client. With these, I'd be able to completely stop using the n900.

The inbuilt terminal is surprisingly good - and even there I have often preferred the software keyboard over the hardware keyboard.

Anything else you want to know?