I was a Nokia user till about a year back. Then I made a mistake.I bought an O2 XPhone II.
Initially I really liked it. Windows Mobile had several features that were really cool and the phone greatly enhanced my productivity.
I noted the following
Positives
Lots of inbuilt features(Like Calendar, Contacts, Email and tasks)
Very very customizable(if you are willing to pay a bit for the tons of software available on the internet)
Plays music and video
Mini SD memory upgradable to 512 MB
Negatives
Slows down a lot of there are applications running
Hangs if you receive an SMS when you are making a call(not in a call but in the process of making a call)
Several calls are dropped(with a message - "Unable to answer Call")
Needed reboots frequently as phone hangs quite frequently - and this is not a soft reboot. You have to open the back cover and remove the batteries.
Joystick is a trouble spot. Use it frequently and it breaks.
Poor service(atleast in Bangalore). Unsuitable response from O2 customer service when I escalated - I went twice to their Cunningham Road service centre after calling and both times I was sent back because they did not have the part(joystick) available
I was very happy with this phone initially but now I realize I made a mistake.
My Nokias were certainly better and more reliable.
If you are looking for a good reliable phone, dont buy it. If you are willing to compromise on the reliablity for the features, you can think of buying.