One of the strengths of the Magento platform is its flexibility and scalability. Customization is common and can be accomplished a few different ways—through community-developed extensions from the Magento Marketplace, or by building your own custom modules. The Magento Marketplace allows you to add new features and themes easily, while a custom module approach might be needed to actualize a unique vision.
Many merchants also pursue a hybrid approach, by customizing pre-built extensions to save time and money. Understanding the benefits of each approach empowers you to make the right decision when looking to extend the out-of-the-box capabilities of your store.
There are several points to think about when considering adding a new extension or custom feature to a Magento installation. The upfront cost can potentially be dwarfed by maintenance and development efforts solving issues in the future. These issues can be avoided, minimized, or at least accounted for ahead of time by choosing a module carefully, and following Magento best practices.