I recently had a chance to work with sitecore cms. Our client requested to upgrade their CMS so they would be able to use the newly added functionality of sitecore.
After we upgraded our sitecore I was quiet surprised that we found a few bugs in their system. The good news is their support team helped us and the bugs are fixed now.
If you decided to upgrade your sitecore my recommendation to you is to add at least a week of buffer to your estimation.