Magento Pros:

• Magento is open source and it is the flexible of modifying or expanding the platform any way you’d like – as long as you have a good Magento expert team to work with.
• Magento has a large community of users, developers, and service providers worldwide. And that community is growing.
• The platform is evolving all the time and there are countless Magento Templates, extensions, modules, and widgets available out there, both commercial and free.
• Whether your store grows overnight or over a year, don’t spend valuable time worrying about your solution not growing with you. With Magento, your site is completely scalable.
• Magento integrates seamlessly with Analytics and Adwords (for conversion tracking), and also has the facility to generate Google Sitemaps to help search engines index your website more effectively.
• The ability to run multiple sites from the same back-end and to be able to manage orders and customers from the same place really makes life easier. Magento also allows you to generate reports based on individual sites, or the business as a whole, which I’m sure you’ll agree is very useful indeed!
• Good supports for multi-language

Magento Cons:

• Magento is a complex, intricate platform built on top of the Zend Framework.1. Lesser Article/Content Management features
• Good Magento developers are rare!
• Magento requires a very good hosting environment and store management.
• A lot of customizations needed to meet practical scenarios (like advertisemnts, templating, content management etc.)
• Detailed understanding of the framework needed to make customization
• Making templates for it takes double time than that for Joomla etc.
• Integration with other systems is not easy