The inventory at Party City is hit or miss, but their saving grace is that if they don't have what you need they'll ship it to you for free if you order on the in-store kiosk. I stopped in trying to find stuff for a costume and came up short, but was able to place an order that will arrive in a few days on the kiosk. It's a little sketchy in that anyone standing behind you can totally read all of your personal information, including your credit card number, but they'd have to be pretty ambitious to memorize it over your shoulder.
One thing that drives me bonkers at Party City is their insistence on playing weird, Kidz Bop-style covers of songs instead of just a regular soundtrack. As a music nerd, it might as well be nails on a chalkboard for the duration. There is no party in soulless remakes of perfectly good pop tunes.
Party City is a good stop if you need supplies built around specific colors, or are throwing a theme party (especially one for kids). For more creative endeavors, not so much, and their costume supplies are barely there in the non-Halloween seasons. If you're shopping last minute you may be left in the lurch (or rather left to go to that great mom & pop costume shop/pharmacy a few miles east your instincts told you to start at), but the kiosk is there for you if you've got some time.