[jdub]

Varnish is extremely flexible. Its primary design goal is to be a programmable reverse proxy cache -- so it's almost certainly a better choice for a challenging session-dependent use case such as e-commerce than repurposed web servers (such as Apache, nginx, etc).

If every single dynamic page of your site requires knowledge of the session, and must hit the backend, then you're kind of stuck. But there are lots of ways to design around that.

For example... Edge Side Includes, supported by Varnish, let you do front-end caching of page fragments, stitched together by the proxy rather than the web application. This can give you a huge boost (to performance and scale), even if you have to hit the web application for some part of the page.

Keep in mind, once you start adding front-end caching to your application, you need to think of your cache as part of your app. :-)

You might want to take a look at the Turpentine module for Magento, which tightly integrates Varnish with Magento. It's a good example of retrofitting front-end caching, cookie generation, and ESI fragment caching into an existing e-commerce application.