Well, personally I wouldn't take them...but I wouldn't use party pills anyway.
With normal medication like panadol etc, the expiry date is a guideline. So long as the seal is intact, then the content of the package should be fine. Its exposure to the air that can cause pills to oxidize and that could change their chemical state.
However, because of the way party pills are produced and packaged, and because you don't really have any guarantee that they have been produced in proper laboratory conditions, you're probably best to avoid them. They may already have had exposure to contaminants during the production and packaging process.
If you have any doubts, don't touch them. Maybe take them to a pharmacist and ask for advice.