We tried a large number of e-commerce "solutions" and most of them had really serious flaws, from promised features that didn't work to simple omissions (like no way to really manage orders or customers). MarketPress addresses all of these and works exactly as promised.
Our one big issue was resolved by the developer. He was downright wonderful in helping us by providing an updated version of MarketPress that solved our issues. See http://wordpress.org/support/topic/single-product-shortcode-not-parsing