Magento is a fairly new ecommerce platform based on the Zend MVC architecture. If you are not a developer; you can interpret this as it is stable and if the custom modules you add to it were written properly, it will continue to be stable.
If you are a web developer just looking to get into customizing magento and have never dealt with a php framework. You are in for a bit of a ride. The MVC architecture is very powerful and once you get the hang of it; allows for easily scaling your modules / applications.
Now that you are up to speed there, something else to note if you’re looking to customize magento easily quickly etc and this is your first time, the database will most likely make your head spin. It uses an EAV structure (Entity Attribute Value). What does this mean? It means its way too big for the scope of this post.
All in all, if you are using os commerce, prestashop, interchange, or any other ecommerce system out there that lacks core features like proper multilingual, coupons, customer groups, AJAX forms and you are looking for more features. Or if you are spending too much time on the instabilities of you Web System; I highly recommend you switch over to magento.
Short pain for a long-term game and satisfaction.