Drupal 7 has been out now (as a full major release) since January 2011. As I write 7.7 is available for download meaning it has already had 7 minor version releases since the major 7 release in January. So - Drupal 7 core is, without doubt, as stable and robust as we would wish it to be.

However, that is not the critical factor in deciding whether or not 7 is right for your new project. One of the major factors is whether or not any of the critical contributed modules (those supplied by the Drupal community) your project will require are missing a version 7 release (or indeed what state that module is in for version 7).

To answer that you need to have completed a comprehensive requirements planning phase and know all the use cases for your website. You then need to have a good knowledge of the major, robust and well maintained modules that will be needed. You need to consider which are available for Drupal 7 - which are not. Is there a full release yet? Is the full release reliable (read those notes and issue queues). Are there any Drupal 7 alternatives for a much needed Drupal 6 module? Is the functionality now in core - like much of CCK and Image Cache? Is there a planned version for 7 or not? Are you willing to invest money in having custom modules developed that are missing in 7?

Just one example. If your project will need to display events with date and calendar functionality - you will find that although the Calendar module has a 7 release right now - 7.2 - the author himself advises you not to use it - but instead the 7.3-dev version! So, for good reasons, a no no for most production sites. Back to 6 then.

So, not such an easy exercise, but, one that should be done before deciding on version 6 or version 7 for your next project.

For me personally, right now, I feel the balance is still with Drupal 6 (and its many many contributed, robust modules) for most more sophisticated projects, but, for simpler websites - 7 is the way to go. Remember, you will at the right point have an upgrade path available to you if/when you feel you need to move the site to 7? Issues there though would be the subject of another post.