If you're willing to shell out a few bucks, I highly recommend mekentosj's Papers over Mendeley. Among the free options, I prefer CiteULike (web-based; http://www.citeulike.org ) or Bibdesk (Mac OS X; http://bibdesk.sourceforge.net ).
I might not be up to date on all the features of Papers but I am pretty sure they never added a better way to sync between computers.
In the last release Papers did add the possibility for citation export to Word, however managing the styling for different journals was far from ideal. I hope Mendeley will solve this for me.