I used the Playtex Dropins system with both of my children because it was refered to me by a family member.  I liked how the liners allowed my children to drink with out getting alot of air, but I didn't like how exspesive the process got.  I ended up washing and reusing liners alot with my second child.  I wish I would have tried different bottles the second time around.  There are much more advanced systems out on the market now. 

My daughter never gave me much trouble with the liners but my son liked to squeeze them so there was alway a big mess.  I think the system is a nice idea, but...