I've heard they are but I have yet to make the financial commitment to buy one myself! Also, because I do not do a lot of baking, although I'm sure it would be useful for other things. If you bake a lot of things on sheet pans I'm sure it would be worth it.
I bought one last year, and I use mine all the time. I bake alot of cookies and they make turn out better (in my opinion) but they are also good for anything that would require a parchment lined sheet or greased sheet. They just slide right off. I am going to get another one .
My mom found me two in some magazine. They were like $11.00 for both of them and they work great.
Check out the Betty Crocker catalog they have them in there too. I have some points if you need them. Just let me know. For a SASE you can have them. :P
I know you've seen them Alix - they are those rubbery sheets that you lay on cookie sheets so the cookies don't stick. Martha Stewart was one of the first I saw use them. They're normally a beige/tan color and some have a border a different color. Did that help?