I built a magento ecommerce site about 1 year ago. Here's a quick summary of my experience:

Pros - Beautiful themes out of the box - Easy to setup initially (i.e. payment plug-ins,etc)

Cons - Extremely slow on a shared host (i.e. netfirms) - If your host is slow, using the admin panel to manage the content is painful

The performance issue killed it for me. Unless you can spend some more money on a good hosting service, I'd stay away from it. Otherwise, it's pretty awesome out of the box.