I've had a string of bad luck using "2-Day Priority" mail as of late. I sent some stamps to PF for certification on 2/27 and it still says "Alert - awaiting signature". Fortunately the PF did get it but the PO never scanned it - I was in a panic for a while. I am also awaiting another stamp from a dealer that was mailed 1 week ago Priority Mail. Tracking says that it made it to our city but has yet to get into a mail bag.
From my experience of buying and selling items, the PO service on "Priority Mail" has really gone downhill. I sometimes think things would arrive faster using regular mail. I sold some lp records a while back and the ones that were sent media mail got to their destinations at least 2 days faster than the ones sent "Priority".
With this in mind, I have a couple items I will be sending PF soon and am considering using Registered Mail. Based on what I read, this enters a different, more secure, stream of mail. Costs about twice as much but may be worth the piece of mind. Anyone using registered mail instead of priority?