Many people will ask "Is the Nexus One an iPhone-killer?" but I don't feel this is really a fair question. To really be a "-killer" it surely needs to do everything the old device did, and in the iPhone's case that means syncing with iTunes. Apple work very hard to lock everyone else out of their software, so that is obviously not going to happen. If you are happy with iTunes then you clearly are not going to be switching to Android.
For the rest of us - who either are indifferent to iTunes or would really prefer not to use it – the Nexus One is a compelling alternative with few major negatives. The UI is responsive to the touch and supports multi-tasking. Coming from the iPhone, the applications are all pretty similar in design and require almost no time to adjust to.
Bottom line: the Nexus One has replaced the iPhone in my pocket as my full-time phone, and I really cannot see myself going back. More than anything, the Nexus One is a pocketable computer that I can use as I see fit.