I've never bought the sport line, but I buy KnitPicks Stroll Fingerling alot and I love it. I use it for my own socks and have no complaints, although I am definitely NOT athletic!
The yarn is so soft that I have used it for baby knitting, too! And for projects requiring a thicker yarn, I often double it up -- I made some great mittens this last winter!
The Stroll line offers a great selection of colors and it's also pretty reasonably priced.
.....hmmmm.... I may have to "go shopping" this afternoon!