I've had a little feedback on SWFUpload. And you might want to thumb through the comments for a few hints (there aren't that many).
Macs (they always gotta be different)
It seems on Macs that if your 'upload_backend' does not return any data that the UploadComplete event will never be called. This seems to happen for any browser which indicates that is probably an issue in the Flash for Mac implementation. So, instead of just having a blank page for your 'upload_backend' just throw in some text like, Thank you. Upload Complete. That, reportedly, will solve the problem.
IOError (specifically, Firefox for Windows)
If you get an IOError then you may not be able to upload any more files. The report says the Open File dialog will no longer open. For me the Open File dialog does appear and the UploadStart event gets called but then I never get any UploadProgress callbacks.
Better Solution: Fix the upload.swf. I realized that the logic in my changes to the Flash file was not very good. When an error occurred upload.swf would just stop processing the queue. Then when you tried to add more files it just tried to upload the erred out file again. So, I've fixed that. There still seems to be occasions when upload.swf just stops responding and you can't get the Open File dialog to come up. You may still have to rely on the "reload the movie" solution discussed above. This still feels like a deeper flash issue that may be part of the immaturity of the file upload features of Flash itself.
Firefox, Cookies and Sessions
I've finally started implemented SWFUpload in one of my own projects. It's been quite the challenge. It turns out that in Firefox for Windows that when SWFUpload uploads the file it sends the Cookies from Internet Explorer. So that means the session set up by ASP.Net (the session_id is stored in a cookie) for Firefox is not re-established when SWFUpload makes the upload. This is a problem I can't fix. I have had to change the way I link the form that my user is filling out and the files she upload since they're submitted separately and I can't rely on any session values.
I also added an upload limit feature. You set the upload limit when instantiating the SWFUpload object and, even if the user queues up more files, the movie won't upload any more files than the limit. I added a new error code (-60) to report back on files rejected because the upload limit has been reached.
Isn't implementation grand!