We use Alfresco to manage digital documents for both our HR department and Student Records departments. We have over 12,000 employees and 85,000 students who's records are all managed using Alfresco and a combination of a custom interface and back end process applications.
Documentation can be challenging -There can be pages of documentation on features and api methods, but they often lack real world examples and are quite often out of date
Early adoption of features can be problematic - Alfresco seems to change quickly and often trying to use newer features can be frustrating. Especially when they keep tweaking/modifying functionality between versions. There has been cases (IE: Records Management) where they've completely re-worked how it works.
Customization of share interface seems cludgy and sometimes limiting
Ultimately a cost decision - Alfresco community is free and has a decent community and various forums, books, etc available. The enterprise version is far less expensive than other options (although costs can add up depending on the the hardware it's running on). It is built upon alot of other mainstream products such as lucene/solr, spring, java and fits into our environment nicely.
It seems to handle a large number and volume of documents. However, if you want to deviate from the 'Out of the box' experience, you will need to determine how much customization would be required and how that would be implemented. (Full blown custom UI, dashlets, java programming, scripting, etc)
We have too much invested into it now to choose a different product. We have been using them from almost the beginning, and have seen decent improvements in documentation, functionality and customer support.