There will always be some degree of fear choosing a product you don’t know. And as Jason points out, sub 1.x rev number scare the least aware whereas it should show that the dev team has high standards and won’t dub their software 1.0 until it’s where they want it to be.
Anyway, the best way to brush away those fears is example : there are many corporate website running MODx and also (show them examples), MODx open doors no other open source CMS has opened, thanks to an unparalleled flexibility. As far as security is concerned, we’ve had very few alerts and they’ve been fixed real quick (let them look at Secunia for example, and our security alert board). Last but not least, the fact that we have a wide MODx coding team is a token of perennity, as is the wide area covered by a strong testing team to report and squash bugs out. The vitality of the community should not hurt either, nor should the ever growing number of MODx extensions.
Pitching MODx to clients is something I have learned to do and, in France, with some level of success against more established solutions such as SPIP, Joomla or Drupal. The "Think Different" pitch work well with MODx, once you’ve gotten past the doubts through live demos (QuickEdit is a winner here) and sample corporate website from all over the world