I have seen multiple projects where Enterprise portals are implemented using Liferay. The main reasons to choose Liferay as a platform for enterprise portal development are -
1. It is open source and cost effective
2. it has OOTB supplort for many common functionality like File uplaod, MEdia, Loing, User Roles and Authentication, which reduces the development time and effectively results in less time to market the application.
3. It supports JSR industry standards like JSR 168, 286, JSR 283 and other many industry standards which helps the application development as well as integration with any third party spftware very easy.
4. Liferay provides very good framework for customization of the portal. it is very easy to understand and implement.
5. The Overall liferay product is designed to be developer friendly. IT does not have any IDE per say. but anyone can use their existing IDEs to create Liferay projects. There is no need to understand the working of new IDE - which is required if we go for other Industry leading Licensed Portal products like IBM, Oracle Portal.
6. Liferay comes in multiple flavours - community edition, entrprise edition - which can be used by any Small and Medium size organizations.
7. There is ongoing support from Liferay community to fix and troubleshoot any issues.
8. The Administrative UI which comes with Liferay is very intuitve and easy to use. Which reduces the Admin work dramatically.
Hence I feel Liferay has an advantage over other Licensed portal products like IBM/Oracle/SAP portal.