USB cable easy to use for now
As it is, using the USB cable couldn't be easier: you get your converted file back from Amazon's free converter service and save the file from your email right onto the Kindle attached to your PC, which looks like a USB mass storage device. (You have to put the file in the "Documents" folder, I believe.)
The hardest part may be that you then have to "eject" your Kindle from your PC before you can see the new document. Anyone trying to see whether a change they made to their doc makes it look better after it gets converted can find those few seconds inconvenient.
But presently it's pretty easy to not have to pay the ten cents, or whatever the new fee is. I suppose we can read a lot into whether it stays that way.
See also the MobiPocket converter, which I haven't tried.
Paris because, alas, I haven't tried her yet either. (No info on whether she's as easy as Kindle's USB cable copying, which frankly would be too little of a challenge for my tastes.)