I am on mac os/itunes. I tried doubletwist, it looked good at first glance but would not transfer album art.

So I tried salling media sync. It worked much better - All album art copied over. Salling in forums states that they have figured out a workaround for the macs to get the album art to the phone. I think they are applying the album art to the file icon before copy.

I would suggest salling over doubletwist.