Hey salam cooldude89,
You're totally right about getting into med. school in the U.S. I think if I tried really hard I could get into a good med. school here, but not without doing a lot of work -- getting really good grades, doing research, getting clinical experience, getting involved in clubs, etc.
I'm not gonna try to say that the best medical schools in Pakistan are close to as good to the best in the U.S. But for me personally, it's a good deal. I haven't been to Pakistan very many times in my life -- and I feel kind of distant from my family/culture. The way I see it is that it's a good chance to get a degree in medicine and kinda get to know the country/the people/my family. Sure the schools over there aren't Harvards or Yales, but all the same many of these schools have been around for decades and are recognized worldwide. They aren't just money-making schemes like many other med. schools these days that are popping up.
I know I'm probably really optimistic about it now and that as soon as I go I'm gonna get that "o crap what have I done" feeling but that's all part of the experience

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It really depends on how you look at the whole thing. If all you're concerned about is medical school, and you're confident in your academic records as well as your ability to do everything that pre-med demands of you in the states, then by all means you should stay in the U.S. Academically speaking, nothing compares to an education attained at a good U.S. school.
If circumstances in my life had been different, and I had been able to visit Pakistan every year to stay in touch with things, I doubt I would have even considered going to med. school there. As this is not the case, I figured going to a reputable med. school there is a good way to pursue my career goals as well as my personal dreams. Also, since I got in through a PTAP seat, it's going to save my parents a lot of money -- $200/year as opposed to around $50,000/year (and that's without consider the cost of undergrad. education which is another 4 years)
As far as residencies go, if you're willing to put in the time/effort and go the extra mile to do electives, do research, and get letters of rec., you can build up a competitive resume for a residency in a good specialty. Personally I'm not super ambitious and I'm not looking to go into surgery. From what I've read/heard, Internal Medicine seems to be right for me but who knows, med. school might change that!
What it comes down to is who you are. If you're looking for the easiest way to get into medicine and get a good residency, I don't think you have the right mindset. It's not easy whether you stay in the States or go to Pakistan. I can't give you much advice regarding residency competition, but from what I've read, it isn't exactly a walk in the park in the States either.
I can't say much about when I want to take the USMLE since I haven't even started going to medical college. I'd like to take it the year I graduate (2013?), but that's something I'll look more into as I start going to school and talking to others that have taken the USMLE/will be taking it w/me and see how things suit me best.
Kind of early for me to know whether I will be doing research/publications. I'd like to. Again, something I can't really give you a definite answer till I reach that stage.
I think prospects for the future are good. I haven't heard of anyone who has tried really hard, gotten good scores, and not gotten a residency here in the U.S. I'm not too worried about not being able to come back, if that's what you mean. If you are worried about that, well, I can't really console you much except to say that a degree in medicine from a reputable school (like a lot of those in Pakistan) will land you a job anywhere in the world.