I've had lots of problems with the Mac uploader, fwiw. Huge CPU use, using less than 1/4 of my bandwidth, not adding to sets, uploading for a few minutes then simply stopping, uploading and then showing me dozens of errors that said a photo failed to upload (maybe one for each photo? I killed it before it had that chance) when in fact they did and were sitting in my photo stream...
The web uploader is way, way better in every way, and far faster, even with the problems I've had with it (if all photos aren't completely uploaded and processed, the 'upload' button tends to fail, but at least it fails atomically so I can just redo everything and not wonder what worked and what didn't).