For certain clients, J1.6 is compelling, especially due to the new permissioning and categories features. These are clients we had to reject previously or use Drupal, because J1.5 simply didn't have these basic enterprise features. As long as we have a decent editor, Extplorer, a few great templates (thanks, Gavick), and some key extensions like Mosets Tree and Ignite Gallery as examples, we're fine.
For clients that need a lot of specialized extensions, like DT Register or JomSocial, or need a vast variety of templates to choose from, we're sticking with J1.5. Right now, our new development is 50/50. Good solid reasons to use either depending on client needs.
And J1.6 has been extremely stable for us, much better at this stage than J1.5 at a similar stage, so the argument that the stability curve will follow that of J1.5 is plain wrong.