This is a complete rewrite of the old Uploadr, using a completely different set of technologies. The biggest visible change is a cross-platform UI built using Mozilla's Xulrunner. The result is an application that looks very similar to the site's recently updated web-based uploading tool, but with many more features.
My favourite feature so far has to be batch tagging, the ability to select a group of images, and quickly apply the same tags, descriptions, and titles (as well as other basic image settings). That's the big win for me - it makes uploading a day's images a lot easier, and reduces the time taken tagging. Group management has been improved too, and if an upload fails, tags and targeted groups are saved for the next attempt.
The result is a big improvement in work flow. I can get a batch of images tagged and uploaded in a few minutes, when using a mix of the old Uploadr and the web UI took at least three times as long...