Ive got a Nokia 6600 at the moment and love it to bits, its the best phone ive ever had.
However, I have voted for Siemens as I had the S25i, M35, C45, SL45 and S55 and every one of them was a workhorse and never gave me any problems. The phones themselves had an exceptionally user friendly interface interface and some of the later ones were actually quite as trendy as the Nokia's.
The biggest problem I have with Nokia's is that most are aimed towards kids, they used to cost the earth compared to other phones that beat them on everything, they are renowned for screen problems/faliures and they are just too late with new technologies on the majority of their phones (although this is starting to change now).