I really like the Merrick brand; they are super hard. Joey gets a week out of them. I held one last night while we watched a movie. For over 2 hours he chewed, and he barely made a dent in it. He's had raw hides before I knew they weren't 100% digestible, and he goes right through those. Merricks are more expensive than some but they last so much longer they are worth the money. Here's a good deal on Ebay, and if your order is less that $20.00, shipping is free.
Merrick Bully Sticks 6in - eBay (item 180307587659 end time Apr-17-09 15:11:43 PDT) Actually, I'm not sure about the shipping, but the price is good for the Bully Stick.