I didn't have any of the 6min solution books, so I can't say much from personal experience. I have just heard that they are a good resource in general.
I took the transpo exam. All I used for the geotech problems was the CERM (10th edition) and a copy of the accompanying practice problems (7th edition). I had taken a geotech class in school, but when studying I found that I had forgotten most of it.
I think the
Practice Problems for the Civil Engineering PE Exam : A Companion to the Civil Engineering Reference Manual had problems that were much more difficult then the actual problems on the exam, but they incorporated much of the subject material into one problem. After working and comprehending those problems I felt very confident on my exam.