I've only been knitting a little over a year but I think I am officially a fiber snob.
My very first project (a pathetic scarf which no longer exists) was with 60% wool/40% acrylic yarn. Ever since then I have pretty much knit only in pure wool or alpaca, except for one lovely scarf that's a cotton chenille.
Well, anyway, I have been working on my little socks whenever I get a spare minute. This includes about 15 minutes during lunchtime at work. Well, one of my coworkers decided to give me a ball of yarn she had at home. She didn't know what it was made of, only that she got it at Wal-Mart (shudder). Based on the hand of it, I'm guessing it's pure acrylic. Knowing I was not going to use that yarn to make anything for myself, friends, or family, I decided to make it into a scarf and give it back to her. Martha Stewart has a nice scarf pattern that I'd wanted to try, so I went for it. It was, well, yucky to knit with. There wasn't quite enough yarn for the whole pattern, so I made the scarf 9 inches shorter than the pattern specified...but it's still long enough to use. What I find most annoying is that the scarf curls in (badly) on both sides, even though it's got a garter-stitch border that is supposed to keep that from happening! I have decided it's because I didn't use wool yarn. :P
Anyway, I finished the scarf last night and ironed it furiously. I plan to give it to my coworker tomorrow. And then I am back to alllllllll natural fibers again. :)
(OK, I do admit I want to try doing something with some ribbon yarn this summer...and that's not natural fiber. And, um, I might find a use for some eyelash yarn at some point. But that's it. Really.)