i applied to many MD/PhD programs this year, and just one DO/PhD program (Michigan State Univ). I was impressed with the program there because it is attached to such a prominent research university. The other programs are not attached to such institutions, and as such, i would be worried about the type of PhD training that one would recieve that such places. I would check with schools to see how they combat that, or if they realize it is an issue on the horizon. Some ven have you go away to another place to do the PhD part.
Also, check on funding. I know that MSU's program is called MSTP, but is NOT funded by the NIH, and that threw me for a loop in the beginning. There are often some fees associated with those DO/PhD programs that are not associated with the MD/PhD or MSTP.
As for getting in, i think very few people apply, and I dont know why. Maybe its a prestige thing, or maybe people fear that a DO/PhD is less competative (once you have the degree) than the MD/PhD. Who knows? That is just my impression.
Any more questions about MSU and the DO/PhD application process there feel free to PM me.
Lynnie