I much prefer to knit with natural fibers: wool, cotton, silk, alpaca, etc., but sometimes I stumble across a synthetic yarn that I like. Some of the things I knit are intended for charity projects and need to be easy care, machine wash & dry.
I bought a skein of Bernat Mosaic yesterday to test knit a hat pattern. The yarn is acrylic, doesn't have any stretch to it at all and is not good for the brioche hat pattern: it just lays there, limp and flat. But, it works up well in garter stitch so I started a diagonal pattern scarf.
I also just finished a brioche hat in "I Love This Wool" for a charity project this fall. The label says hand wash, but I am going to do a test wash with the hand-wash cycle on my Maytag washer.