Even if only the uploader has the appropriate key, he can still pass on files with simple links containing an access code. So the person who calls up such a link does not need a key. It can be assumed that if MEGA.co.nz is successful, pages will soon appear that collect such links and thus help the holiday videos to reach a larger audience. The start seems to have been quite successful already: according to Kim Dotcom, more than half a million users have already registered after only a few hours, and the servers are difficult to reach even at the moment. At the moment MEGA is still running as beta, the provider recommends Chrome as the ideal browser.
What remains exciting is how the music and film industry will react and whether users will accept the offer at all. The question remains as to why one should entrust someone facing proceedings for copyright infringement with their data - but perhaps the user group addressed does not care. If you want to keep control over your own data, you should have a owncloud installation think.