I have been using chemical toe warmers in my cycling shoes for for a while and really love them. They are just the thing to keep your toes warm. I've been using the Grabber brand which are thinner than the little Hotties brand. (Just bookmark that link. Little Hotties is not something you want to google for.)
I picked up 8 pair of the Grabber toe warmers for $11 from www.joessports.com (12/2008). That's not a bad price, but it's just enough for me to make sure I really need them before opening up a pair. So I was tempted when I heard that Costco was selling a box of 40 pair of hand warmers plus 3 pair of toe warmers for $16 (1/2010). Any time I can get something for a third of the price, I'm all for it.
Well, you do get what you pay for. The little Hotties are great hand warmers, but they are less than ideal as toe warmers. They are the thickest of the three products shown here. I was able to put this over my toes and ride without thinking too much about them. But the flatter product is better. I'm not sure how the hand warmers would feel under my toes. Haven't tried it. Not sure I want to.
The toe warmers also have an adhesive backing which helps you get the warmer where you want it and keeps it there. And, they are larger, which helps spread the heat better over your toes.
Bottom line: Chemical hand warmers are a cheap and usable substitute for toe warmers, but they are not as enjoyable or effective.