They work great. I made one from some packing that we were tossing at work. Covered it with heavy plastic and everything slides well. Cost me almost nothing to make. I do, however, occasionally spray the plastic with silicone spray and wipe it off. Then things really slide around.
Pluses: Inexpensive, works well, you can make it whatever size you want (eg. to totally cover the table you are using to sew).
Minuses: Not easy to transport, but anything that size wouldn't be easy to transport.
Hope this helps.