I have a medium vortex and just ordered a small, plus another medium for my dad. At $30 something bucks, it isn't cheap, but it's extremely high quality, heavy gauge stainless steel.
If you want to get one, I would recommend the small for use in the 18" and 22" - my medium is a little too big for the 18" kettle, but the small will work fine in the 22". I'm a really big fan - if you are on the fence, I think it's a worthwhile accessory.