Since 1.5 is now stable, we plan to finish converting our extensions soon. Some of our extensions, such as iJoomla Magazine and iJoomla Surveys, are almost done. Others have some way to go.
But we still don't recommend that you upgrade to 1.5.

At the moment, there is no simple way to make the migration. That might change later this year, and if you're starting a new site, you can consider using 1.5 as long as you've made sure that all your favorite extensions are available or will be soon. But even for a new site, 1.0.x is still a good choice. It will take time before the Joomla community adopts 1.5 and some people will never upgrade.

There is a good blog entry about this topic, you can find it here.