I've been doing a lot of research on enterprise search products lately and one of the biggest issues is security. Unleashing search on corporate systems where permissions aren't managed really well is a big risk.
Using the free versions of all three (Solr + Drupal + Alfresco), search is fast and access-controlled very easily.
I worked for a company that had a Google Search Appliance about 6 years ago, and, like others have said, it basically sat in a rack somewhere and collected dust once we started using some form of integrated search. The GSA was fast and relatively easy to use, but required an expensive initial purchase + ongoing licensing/update fees.