You will likely find that anyone who switched from a 510 to a 901 had their hubby grab the best one and hide it.
A big hint. If you think your 901 is better than a 510 just try to trade a 510 owner out of one for a 901. I have nothing against 901s in the slightest, but they simply do not compare across the board with a 510 -- not even close.
About all I'm seeing (negatively) about the 510 is the battery life complaints. I don't really have a problem with the battery-life... or I'm simply used to it. Maybe I'll just work on increasing my 510 accessories for now. Still, I might keep my eye out for a good bargain on a 901 in the future.