Never liked any of the Sony Ericsson's I had but was using that X10 and its much better than the Iphone, double the battery life, better camera, multitasking, plus its quicker processor. Having not owned one but I can't recommend it the build quality of Sony Eircssons has never been good.
I have an N97 and I wouldn't recommend that over an iphone, the HTC phones have questionable build quality as well...BUT I couldn't live the battery life of the iPhone at the end of the day its supposed to be a mobile device how mobile is it when you need a wall socket to use it for a day?