As with any software, a new release often has bugs in it and it takes a couple of months to get it stabilized. In addition, it takes time for the Joomla website development community to upgrade existing extensions. There are over 6000 Joomla extensions! Granted, the most reputable and popular Joomla extensions do upgrade quickly, but this is software, and designers are only human, so mistakes always happen, bugs are always present, so it is wise to wait until the newly released version gets stabilized. 

With all that said, upgrading from Joomla 2.5+ to Joomla 3.0 is not difficult. And futher, from Joomla 3.0 to its subsequent sub-versions is promised to be a breeze.

If you have a Joomla 1.5 site, it is necessary to upgrade to Joomla 2.5 first and then from it to Joomla 3.0. The only alternative is to re-creare your site anew using Joomla 3.0. If your site is more than 3 years old, this maybe a good excuse to get a new website.

In other words, with new Joomla 3.0 version, upgrades should be fast and easy and can be done from the Joomla admin back-end. Joomla is saying they will be releasing a new version every 6 months. It will be hard for the extensions community to keep up with these new versions, so upgrading once a year should be sufficient. Note that not each new major Joomla version is good to upgrade to. Sometimes it is best to wait until it matures.