Granted there are still some basic features missing on the n900 that are available on other phones like MMS, but PR1.2 has made some improvements. I can't comment on the speed and performance because i've reflashed my phone and plan on starting from scratch, so the phone is obviously performing really good.
However, the USSD commands are working now and have long been a sought-after feature. Also, I'm really enjoying the new ovi maps, just feels better. It got a lock in less than 10 seconds while i was indoors.
What else... QT4.6 pre installed, which is supposed to be big news as i've gathered from the many threads here (i've personally haven't benefited from this yet but there's a first time for everything )
The virtual keyboard looks and works great, and weirdly enough, after the update i can now access the full facebook website rather than the mobile version (which was a problem that arose to all n900 users a couple of days ago if i'm not mistaken). Another welcomed feature is the share option from the file manager. This allows you to share files directly from the manager via bluetooth, email, and other services.
I still haven't tried out the camera to see if there are any changes, but settings-wise, they've added more resolutions to both image and video capture as well as night mode for the two.
People also say that the email client has also improved, but i haven't tried that myself either.
Just try it out, give it some time, and i'm sure you'll start noticing and maybe appreciating the changes, as subtle as they may be.