Speaking as one who hasn't yet started his EM residency, and one who will never do an IM residency, my advice is worth a little less then it costs, but here goes:
EM/IM folks really seem to know their stuff through & through. They cover the two opposite ends of the continuity spectrum, and occasionally are able to apply knowledge from each end to the other. Good for them.
I considered doing the combined residency thing until I realized a few things:
1a-I want to be an EM Doc.
1b-I do NOT want to be an Internist.
1c-One can be a great EM Doc without ever doing an IM residency.
Taking these together I realized that a combined EM/IM residency would equate to little more than two years of mental masturbation for me. I aint knockin IM here - it's just not for me. And I aint knockin mental masturbation either, I'd just rather ruminate on Relativity, Wittgenstein, or Punctuated Equilibrium than IM.
2-Those who are great at what they do are usually, if not always, focused on it.
The day of the Renisance Man is behind us. Our body of knowledge is simply too large for any person to master it all. As such, I think that, while an EM/IM person is likely to be quite able to flex his/her knowledge on rounds & really impress those M3's, one who practices two fields will be less strong in either than he or she would be if his or her efforts were focused on a single field.
3-One of the main benefits of EM/IM is the academic doors that it opens. I don't want to do academics (thank goodness there are others who do!), so the pay-off just wasn't there for me.
4-EM/IM residencies include about two extra years of rounding - 'nuff said.
But if you feel differently about IM than I do, then your views might be in stark contrast to mine. In fact, they almost certainly are.