This was long time coming: our portal was aging, the CMS running it was out of date and since we specialize in Joomla development, we needed to make the jump, bring on the latest and greatest version and see how the migration actually works.
I find it refreshing to see how good Joomla is. The migration from 1.5 to 1.6/1.7 is time consuming, but that's the biggest hurdle. Otherwise, it is uneventful, easy and safe. Some of the time consuming activities come from setting up the safety net: backups, backups, backups. If you have a very elaborate template, you might have to do some work around it to bring it up to date for the new system. The rest is simple: data seems to move without a glitch, which is the most important part, as you don't want to re-type all those blog entries.
Joomla! 1.7 is a gradual improvement from the previous versions. However, since Joomla has been an exceptional product for many years, the gradual improvement means the current version is safe, easy to use, extensible and flexible for the end users and administrators. As always, I would highly recommend it for any Web based project, from small to large.