As someone doing a DDS/PhD at UCSF, I would add that it's really important to know what you will do with that PhD, should you choose to do it, in conjunction with a DDS.
What was once a defining 'graduate' degree (the PhD), is now, like many other professions, an intermediary, leading to a post-doc that is more final and critical. The decision to do a PhD is not light, because with a 3 year postdoc that typically follows, you'll be having a hard time realizing why do the 4 years of DDS?
Should you not want to do a postdoc, and not do more science after the program, then you'll be wondering why you did the PhD?
Certainly, you could be the DDS/PhD that does both a research postdoc and a dental specialty afterwards...
I suppose you could retire when you are 'done'. 😀
Bottom line of my message: if one is planning the DDS/PhD route, plan ahead!