Take it from someone who has been there, done that.
I did NOT take the USMLE, just the COMLEX. I am applying for the very competitive field of Emergency Medicine (just check out the EM thread here)... and so far have 19 interviews at very good programs (Univ of FL, Univ of MD, Case Western, etc). I did not and never will take the USMLE.
Throughout my medical education there were always two camps that talked about the USMLE.
One camp said:
Why take the USMLE? If they make you take the USMLE becuase you are a DO and they don't care/understand the COMLEX, they dont' care/understand about you. The COMLEX is standardized just as the USMLE is. One reason they do not accept the COMLEX is because they are not as DO understanding/friendly... followed by the "if they want you to jump through that hoop, you wouldn't want to go there, there's no need to do that."
Other camp:
You need to take the USMLE to be on a level playing field.
I bet there is an element of truth in both statements, really. I think the better answer, is that if you did only so-so on the COMLEX, you better bust yer balls or ovaries and do really well on the USMLE, to prove yourself.
I talked to a program director for an EM residency in the Northeast, and asked him his opinion... he said it didnt' really matter.
Now to specifically answer your questions:
COMLEX is a hassle. Two days, four books, 200 questions each.
I would guess that all MD students take the USMLE. So... roughly 100 x 120 or so MD schools? There's your answer.
Taking the USMLE is much nicer than the COMLEX. Just one day. Computerized. Your own pace.