I'm in the MedSci master's program now, and you have two options for applications: assuming you apply and get into the master's program, you would start the program May of 2017. You can either apply for medical schools the same year you are doing the program, or you can wait until after you complete the program. If you apply during and get in, you would start medical school in july 2018 after graduating from the master's program. If you decide to apply after completing the master's, you would be starting july 2019 assuming acceptance.
We have students doing both options; most of my fellow master's students are completing applications and interviewing at schools for the next entry cycle while doing the MedSci program, so that we start medical school in 2017.
If you do well in this master's program, you are indeed competitive at MD schools. This is a graduate level program, and the classes are faster and denser than undergrad. If you do well here, it makes up for a lot of "sins," if you will, from undergrad. Most schools have told me you need to get a 3.5 or higher cGPA in this program to show you can handle medical school curriculum, so you still need to do very well (mostly As, a few Bs here and there are acceptable). Some of the MedSci classes are set up in 6-8 week blocks, so that once you finish a course and the grade posts, you can send off the updated transcript to TMDSAS/AACOMAS/AMCAS to show the schools your new scores. I know a ton of people in the program who have gotten into MD schools already.
What you decide to do as far as medical school applications go is entirely based on your own personal situation: for me, my cGPA and MCAT score were very competitive, but my sGPA was lower than most schools liked to see, so I enrolled in the MedSci program to help bolster that and applied to medical schools simultaneously. I've also been out of school for about six years, so I figured this program would be good to get used to do full-time school again.
Have you applied to medical school already? If you have and did not get any response, I would suggest doing a file review at a few schools so that you can see what the deficits were. This might help guide your decision about applying to medical school, assuming you are enrolled in the MedSci program or something similar.
Hope that helped!