I ordered the Treads and Sprockets Pack from LEGO Education a few days ago, and it just arrived today. I was quite surprised to see how many pieces came with the pack - there's a lot! As you can see in the picture above, there's enough to make four good-sized sets, which is probably more than I'll ever use. As you can see in the picture on the left, I put all the links together and strung them out in one long chain... it's really long! The NXT Brick is used for scale.
My only dissapointment with this pack is the fact that it only includes the small sprockets and none of the bigger, yellow, ones. I don't have any of those and they would probably come in handy. But overall, I think this is a great deal for only $12.00 (minus shipping) - it includes 10 small sprockets and 100 tread links.