VSS Converter is a far from perfect solution. And there are significant differences between the 2005 and the 2008SP1 version of the converter.
For example, in a VSS DB that's been in use for a long time, there will have been a large number of users contributing to VSS. Many of these users will have left the organisation a long time ago and therefore will no longer have domain accounts. TFS requires mapping VSS users to domain accounts, so you will have to decide whether you map old users to a single 'dummy' domain account or to a current team member.
In addition, VSS Converter 2008 requires these domain accounts to be valid TFS accounts. Whereas the 2005 converter does not enforce this.
If your VSS history contains significant folder Moves, then it's likely you will loose all history before this Move. For example, if you Move a folder to a new location, then Delete the previous parent, you will loose all history. See this article for more explanation:
http://msdn.microsoft.com/en-us/library/ms253166.aspx
In one migration I was involved with, we had a 10 year old VSS database that lost all history prior to 6 months ago. This was due to a significant tidy up that took place 6 months ago.