Magento

Actually the demos are pretty slick, but the code is nearly impossible to modify, since it is all based on obscure Zend framework stuff and abstracted to death. (Hundreds of folders, nested 7 deep.)

Magento is not a piece of software you just install in a couple of hours and customize in a day. You will probably need an e-commerce specialist/ Magento partner to implement the software for you. But if you have it running, connected to your ERP/CRM/Financial software and using it's great marketing features you'll know it's the most versatile, powerful and profitable platform you can get. Open source or otherwise.

http://www.magentocommerce.com/getmagento/1.5.0.0/magento-1.5.0.0.zip

After 2 weeks of use / dev :
Slow (requires dedicated server)
Poor doc
No (or wrong) phpdoc
Over-engineered and ugly architecture
Some crapy code
Takes ages to develop simple things
Long way to learn how it works, and how to extends things.

If you don't have the choice, buy php|a's book (http://www.phparch.com/c/books/id/9780973862171)