Not sure if this is still relevant, but at some moment they acknowledged their stuff access to client's data. This is huge no-no for any privacy conscious person.
Other that that it's a great tool for organising personal information for me.
It's also really versatile. You can use it as a wiki, as a document hosting platform (like google docs), etc.
I add Google Calendar to it for time-based organization and Google Contacts for people knowledge. (I really wish this existed: A tool to aggregate my conversations across multiple mediums such as emails, tweets, DMs on various services etc and tie them to my contacts in Google Contacts.)