I noticed Jottacloud on the Drop Dropbox thread and gave it a try. I've been running it on Windows for the last couple of hours with a copy of the files I used to keep at Dropbox and the only difference I noticed so far is how Jottacloud uses around 25% (~16Mb) of the RAM Dropbox was using before (~64Mb). Can't say it makes any difference on my MacBook Air.