I've swapped some as well... around 35 times or so? I have never swapped for mascara and only (supposedly) new wand lip gloss - a couple have looked "tested" but I wiped 'em off & tried 'em anyway... and like Halo, I was fine!
I like swapping because it gives you a chance to try something out before actually buying, especially items that you may not get to see in person (like the high end items us living in Podunkville don't have access to and have to purchase online). It's also cool to send something on to someone else after I've tried it & decided it doesn't work for me.
I like swapping for bath stuff (gels, soaps, bubble bath) and lotion, perfume, blush, eye shadow and lip gloss mostly.
Swapping is a lot of fun, too, because a lot of people include "extras" which can be anything from samples (or even full-sized items) to tea light candles, votives, tea bags, stickers, barrettes, pencils, and candy. Sometimes you get a package so chock full of stuff it's just like Christmas!!