How is Magento when it comes to adding custom functionality?
How is working with Magento when it comes to adding custom functionality?  

From an outsiders view the framework looks very complicated and it's hard to gage what is and is not possible.  What are your experiences?  Is it a difficult framework to get it to do what you want? Are there things that simply cannot be customized in Magento?  How difficult is it to manage customizations when upgrading to latest versions,etc..

Any feedback/personal experiences would be great.  Examples of highly customized Magento sites would be great too.  Thanks for your help.
How is Magento when it comes to adding custom functionality?
===

How is working with Magento when it comes to adding custom functionality?  

From an outsiders view the framework looks very complicated and it's hard to gage what is and is not possible.  What are your experiences?  Is it a difficult framework to get it to do what you want? Are there things that simply cannot be customized in Magento?  How difficult is it to manage customizations when upgrading to latest versions,etc..

Any feedback/personal experiences would be great.  Examples of highly customized Magento sites would be great too.  Thanks for your help.