I have used opencart, prestashop, and pinnacle cart.
OpenCart and Prestashop are very similar, both with fairly clean php code and nice ajaxy themes. I moved from Prestashop to OpenCart only because Prestashop doesn't support Paypal Payments Pro ( basically Paypal not in the normal way, but as a traditional CC processor). I liked both of them. Both of them do have the same hassle of having to use "community-provided" plugins and extensions for things that you might assume would come with the cart. Some of the plugins are high quality, and some are crappy. They aren't all free either.
I use pinnacle cart not because I picked it, but because I bought two sites that already had it. It's not bad, and certainly not as messy as some of the older shopping cart scripts, nor as bloated and slow as magento. It is, however, expensive. The "run on your own server" price, including source code, was $1k. Ouch. If you don't need the source code, you can find hosting with the cart included for about $30/month. The reason you might choose something like pinnacle cart is that it does most things out of the box without having to download plugins, etc. ( PP Payments Pro, Canada Post shipping, Google Products integration, etc)
