- Free and open-source. Very actively developed and new updates are coming out very often.
- Easy to install. Just grab a spare (networked) server, VPS or even just a shared hosting account, install Apache/NGINX, PHP and MySQL, unzip and off you go.
- Privacy. Using this means that you have full control over your data rather than trusting a third-party like Google or Microsoft. OwnCloud even supports encryption of files. Enables businesses to more-easily comply with data protection laws while having the cloud convenience.
- Extendable. Lots of plugins available to integrate with OwnCloud and the ability to make your own as well of course.
- Strong user management/group functionality to share your cloud with multiple users. Ability to integrate with Active Directory and other similar products.
- Desktop sync client and mobile applications really make this feel like a complete product.
- Enterprise support available if needed.
- Performance can be pretty poor at times, especially when dealing with large amounts of files.
- Some features are still rather buggy/unfinished. It's constantly in development though so this is to be expected.
If you're looking to host your own alternative to Google Drive, Microsoft OneDrive etc. you can't go too far wrong with OwnCloud. This is the most complete alternative in a box that I've found so far.