They didn't come out with the drop-in liners until long after my first daughter quit drinking from bottles.  But, I was introduced to them when I had my second daughter this year.  These are the best items EVER!!  It's so easy, you drop the liner in, fill with formula, put the nipple on and you're ready to go.  Not to mention, they're easy to pack and you don't have a huge mess to clean up.  You just simply take the liner out, wash the nipple, and you're done.  What could be any easier?!

The only thing I don't like about these, is that they are fairly expensive.  However, if you buy the store...