Personally, I wouldn't waste my time with comment cards. I would ask my customer for feedback. Getting people to give you comments that mean something is hard. And they are at someone's party and taking the time isn't going to be a priority.
Once they leave the place they aren't going to remember to go to a website.
I think your best solution is getting your customer to provide feedback. If you are dead set on getting feedback ask the host/hostess for the names and phone numbers of a couple of their quests and call them on the phone.