They're great to give out to other people, just don't give them to anyone in your immediate vicinity. They can get annoying. And warn people not to leave them near computers or credit cards because they are extremely magnetic and can cause problems. And I can pretty much promise that a customer will not give you business if your tradeshow giveaway ruins their credit card or computer.
Bags are very popular, especially because everyone needs them when walking around. Food is good, but once it's eaten, your logo is in the garbage.
Personally, I'd go for items that will engage your attendees. Noise is good, but nothing that's too loud. Light-up things can be fun. I tend to love scented highlighters. Any booth that has them when we go to expo's gets my attention.