I have some pretty serious comparisons to make in the next few months...
I've had SE phones on AT&T for the last six years (and an Ericsson phone before that), so I am really looking forward to the X10 soon coming to AT&T. To me, it's a foregone conclusion and my first choice. I'd like to compare this to the HTC Desire, also rumored to be soon coming to AT&T. I just hope AT&T doesn't cripple them with their crapware. If AT&T does "ruin" these with their crap, I may end up looking at the HTC Supersonic on Sprint. And there's always the Nexus One.
But I REALLY want the X10, hands down.