The only problem I have had with my phone, which is a Sony Ericsson Z750a, is that it shuts itself off at random times. It's happened during phone calls, when I'm on the Internet, sending a text message and even when I'm not using it.
I've had AT&T look at it three times, but it hasn't been fixed. But I figure I can wait unitl December 27 or whatever when I can upgrade. I have no plans of getting rid of AT&T -- been with them for five years now -- and no other company works that well in my hometown. Verizon and a few other companies work where I work, but none of them have very good service at home.