Why Magento?

Open Source. Open Source means your store and the platform its built on are fully yours. You have the freedom to customize Magento, tailor it to your business model and expand on it for years to come. No strings attached.
Integration Friendly. Your eCommerce platform should not dictate what third-party solution you use. Magento gives you the freedom to work with the third-party solutions you want.
Feature-Rich. Magento comes out-of-the-box with features like one-page checkout, product comparisons, multi-address Shipping and many more features that will set your store apart.
Support Network. Magento is fully supported by Varien and the international Magento community ensuring that where ever you are, you’re never on your own.

source:http://www.varien.com/products/