I can only speak from my experience, but I'd say doing those tests are well worth the time. Unlike many others, OMM is not all that intuitive to me, and I thought doing those tests in the back really helped me turn what could have been a significant weakness into a non-issue (or at least so I felt...still waiting on my score).
The difficulty level of the questions in the back is a fair bit above the level on the COMLEX, but I thought they prepared me well for virtually everything OMM-related I saw on my exam.
Good luck!