Just got my white L900 that replaced my Focus 2. This is one nice piece of hardware. I like it better than the iPhone 4 I have. I can't say a single negative thing about the hardware.
Saying that, and I am running this side by side with my Focus 2. The processing and screen/touch sensitivity seem...off. Sometimes I'll hit the back button once and nothing happens, then I tap it again and it opens up the multi-tasking windows. It takes a bit of time to get used to, I've learned to use this phone a bit differently than the Focus 2 and Focus. After a short period of time, I actually checked and double checked to make sure I had all of the updates and I do.
Overall, I am extremely happy but the actual UI and processing doesn't seem as smooth as my other WP's.
Update: Glaring issues are starting to pop up as using the phone more. The buttons on the bottom of the phone are extremely frustrating. Hitting the back button while playing almost any game opens up the multi-tasking windows. My phone also has the dreaded vibration rattle - it's frustrating. I had to turn off vibrate notifications.
I am still pretty happy with this but after doing some research, Nokia has known about the input lag, button issues, and vibration rattles for months with no fix as of yet.