Is Drupal Commerce ready for the real world? I'm trying to make a decision between Drupal 6 + Ubercart and Drupal7 + Drupal Commerce... I'd much rather develop in 7 and am excited about Drupal Commerce but, as it is relatively young, have some concerns about its security and stability.

Does anyone have any relevant experience they would be willing to share?
The payment gateway is the key. If your chosen payment gateway is available for Drupal Commerce, I would say yes, go ahead and use it.

Even if not, there are compelling reasons to develop or help to develop the payment gateway.

Drupal 7 is much more advanced, and will continue to be relevant for much longer. Drupal 6 will not be supported when Drupal 8 is released. Suppose Drupal 8 is released in 2 years (nobody knows at this stage but it's a reasonable estimate) and your site takes 6 months to launch. A Drupal 6 site would only have a viable life of 18 months. It's something worth considering!

We're building a reasonably complicated Drupal Commerce site right now and the elements like the order system, cart, checkout and things like entity types are all fairly concrete. It's also slick to use, and offers numerous advantages over Ubercart (products do not HAVE to be nodes, opening up new ways of selling things).