Obviously, you will start out with a wonderful grasp of pharmacology. Also, it is nice having at least been exposed to a variety of health conditions and human physiology. That being said, there are a good number of subjects (histo, embryo, anatomy, etc.) that you will be weaker in than those who have bs/ms in various biological fields. In the end, it is a tradeoff; however, I am very, very glad I spent some time in pharmD school.
That being said, getting a pharmd, just to prepare oneself for medschool, is a big waste of time and money. Spending some time in pharmacy school (or if you can find a rare pharmacy program that offers a bs in pharmaceutical sciences) is a great way to go.
I can say that anyone telling you that you will breeze through is indeed blowing smoke up your ass. Medical school is much, much more challenging than pharmacy school. I remember back in the day hearing some students in my pharmacy class debate whether pharmacy or med school was harder...I wish I could go back in time. For example, I took a very tough semeser of biochem in pharmacy school; when I entered medical school we covered everything in MUCH greater detail, plus much more, in the first 4 weeks of biochem! The head of the toxicology department where I attended pharmacy school told me that the immunology course he taught in pharmacy school (again considered to be a kiler course) would be covered in less than a month in med school. He was right. Having been to pharmacy school, I can say that you have an awesome foundation, not only in terms of material, but also in terms of being in a professional school and having a good study foundation.
Hope this helps,
Jason