Personally I have not done that migration because we have a deep customization in our project (SM 7.01) so it is not possible to do the migration at the moment, but my colleagues have done that migration with the HP guides and then say that it was successful. I can not tell you if they had a deep customized system, but as I know it was finished without an errors.
Using the demo db, the review report was generated in 20 minutes or less. I suggest you get generate a review report off a copy of your production db, to get a better feel of how much change is required. Sometimes, trying to find the reason for a customisation and whether it is still valid for current business may end up consuming more time than expected.
We have a fairly significant tailoring in our system and it took me about 2.5 days to do the whole thing and generate the custom patch. One thing to keep in mind though, is that HP released this without fully testing the mechanism. It is possible to create the custom patch for migration up to test and prod on one time with out errors. If you get everything right the first time, this is not an issue, but if you find issues that need to be resolved and attempt generating a new custom patch, it will fail. I worked with support for about a week on a work around, but could not get a tested/proven work around in time to move forward. Had to pull the patch from our current release.