1) Make sure all upgrades are completed as requested (Joomla higher than 1.5.19 - I upgraded to 1.5.25 first) before starting the upgrade
2) I did experience the "freeze" in IE, but it worked in Firefox
3) After the "migration", the new site is now at www.yoursite.com/jupgrade until you move the original files OUT of the root and the file in the jupgrade directory into the root. (there is a great post in the comments of the upgrade video on youtube on how to do this)
3) You will need to "clean up" the new site, as I found the JA Purity template from v1.5 had some differences from the default v2.5 templates, so plan some time to redo some banners, etc.
Other than that, it seemed to be a good process. Not including the "tweaks" to the new template, it took me less than 2 hours for the entire project, including research.