Let me tell you how much I LOVE this component. I was hesitant to upgrade Montano Designs to Joomla! 1.5 because of the utter hassle I thought it would be to convert all of my content and links. Not so with MTW Migrator. It was a simple as creating a new installation of Joomla in a sub-folder, installing the component and running it. If you use Community Builder or VirtueMart you'll need to install those prior to running the tool. You'll also need to install any modules you have on your old site you want converted to the new site. I didn't use that option, but keep in mind that the module has to work in 1.5. Otherwise it won't do you any good to pull the information over. This tool doesn't install the old modules, it only tells Joomla where to put them.
A security side note: It is wise to uninstall any non-core components, templates and modules if you are not using them. Developers try to make them secure, but the bad folks out there are always trying to find a way to exploit websites.