A used Nokia N95 from a guy at work (just as proper smart phones were coming out) which I then left on the roof of the car as I drove off.
Not exactly buyer's remorse on my part but I remember around that era having a Sony Ericsson phone which was amazing and raving about it, suggested the same line of phones to one of my colleagues and kind of suggested towards the replacement models when he was looking for a new mobile - he went out and bought the most expensive version and hated it. Felt a bit guilty in a way as it turned out they'd downgraded some features in the replacement models.