I've used GoogaLoom, PayLoom, and now RapidCart. RapidCart blows the other two away, because it doesn't require users to interact with an unexplained pop-up PayPal window every time they add something to their cart. Instead, items' icons swoosh up into the cart icon, and all the shopping cart functions are integrated into the page, hovering there (through Ajax or whatever magic is running it).
I switched to this plug-in and am very happy with it. I think it provides a much more graphically polished and pleasing look on my site, and must inspire more confidence in users because of the visually self-evident way it carries out the shopper's selections.
It also allows for pop-up menus that change each item's features; my only wish is that these could affect the price (so I could have users choose local or international shipping, for instance, instead of having to host two separate shops on my site).