I am currently working as an EMT-B(with my IV cert) and I have loved the work. I started working 2 years out of college in an attempt to learn more about healthcare, find out if I like health care, and to 'pad' the med app in the case that I end up going for my md. I have been working for 9 months now and I have found a real love for the excitement, diversity and weirdness of the job(what other job lets you watch movies and read for hours at end, and then all of a sudden drive like a maniac to go push needles and pieces of plastic into somebody!?) I will be starting MD school in the fall and I regret not being able to work longer as an EMT.
Ok...to answer your questions. It seems from your post that you are confusing WORKING as an emt with VOLUNTEERING as an EMT. If you volunteer as emt, you might be 'on call'. Volunteers are typically found in rural settings. Since I a paid emt, we hang out either in our station or we are posted at a location, in the ambulance, for a full shift(12 hours in my case). Keep in my mind-you have to be 21 to work at my company. This is not with all companies though(I think you can work for AMR, a national 'walmart' of ambulance companies, at 18. Double check on that one though).
Since I never worked during college I can't say what it is like. If you can work part time for the agencies in your district, then it seems like a great job! The pay does suck though(Since my company is smaller, I am paid on the lower spectrum...a mind blowing 9.60 $/hr). Depending on how busy the system you work in is, you could spend hours at a time doing nothing-which means you could be studying and doing homework.
As far as how EMT work looks on an app...I got in, so it didn't hurt! Admission committee's are looking for 1)how much you want to be a doctor 2)what you have done that shows you are committed to medicine and 3)that you have an inkling of what you are getting into. Working as EMT provided me with a foot in the door of healthcare. I did an intensive 2 week accelerated program(PM me for info) that meant I got it done relatively painlessly. I think being an ER tech is a better way to learn about healthcare, for you are working in a hospital and also working closely with MD's. These positions are harder to come by, unfortunately.
As far as activities that look good. Try and stick with things that you are passionate about. I know some schools wouldn't touch applicants with a ten foot stick if they have no volunteer ec's...so volunteer somewhere at some point. But for interviews it is hard to impress someone if you don't feel strongly about something-so you have to have at least LIKED what you did!
Good luck and feel free to pm with questions.