We use it for scanning large volumes of documents into SharePoint. It is fast and usually accurate. The OCR software they use (ABBYY) is fast and can recognize some text I had trouble reading. Their support staff was very helpful when I had SharePoint dumped in my lap when the previous guy left. Our internal customers are happy with it for the most part.
Upgrading from SharePoint 2007 to 2010 was a bear. This is not specifically a problem with KnowledgeLake but since it is built in to SharePoint it had to be fully removed and all users migrated at once then re-installed. Also we had some users that were using an older version of their client software. They could not use it at all (lesson learned). :)