Well overdue this review after using it with so much success! After the frustration with other tools available we tried a few years ago SP Upgrade as soon as it was released. The suite is excellent and has matured over the years into a very reliable and rock-solid tool to migrate any website from Joomla 1.5 to Joomla 2.5 and/or Joomla 3.0. We are a large global operating, commercial Joomla support organization and we do many, many migrations of all sorts of websites. Every core Joomla migration, small or large is done utilizing SP Upgrade. The largest site we have migrated using this tool was a Joomla 1.5 website with 90,000 articles and a database of 720 Mb. Smoothly and without any loss of data. Links perfectly intact and no loss of Google ranking in the end. The documentation is excellent and is proper updated. The costs are reasonable and you pay about US$ 40 for the component but it is worth the money since you migrate in minutes an average Joomla core site where the "good old" free tools simply won't do the job as good as SP Upgrade (do your research on the Joomla-forums and extension reviews) and they take way more time and effort and repairs after things are completed.
You want to migrate? This extension is the way to go (!)