I love this swim diaper.  I got the Medium, which is for 18-22 lbs.  It fits my 18.5 lb son just right and it has a nice soft layer inside against his skin.  I also like the fact that I am not wasting money, and landfill space, on disposable diapers.

Updated June 2011: I liked this diaper so much I purchased the Large (22-25 lb) so I have it when my son outgrows the medium size.  He is 21 lbs now so he'll need the large size soon.  This is such a great product.  I tell all my "parent of younger children" friends about it.