I just graduated from Army ROTC and will be starting medical school with HPSP next month. The first thing I will tell you is that it is VERY competitive to get an educational delay right out of ROTC. The Army has certain slots it has to fill in every branch. Some branches such as aviation, infantry, armor, and medical are very competitive. When you are in Army ROTC you are ranked against EVERY OTHER CADET in the nation for your branching (I can't speak for navy and af, but I assume they are similar). Your branch depends on how you ranked in the nation, how other cadets ranked who also want the branch you do (as they are your competition), as well as needs of the army. For example, with my year group, over 100 applied for educational delays and less than fifteen got them for medical school. I got one because I was cadet number 62 out 4450 in the nation, and top of my school. If you think you can be in the top ten percent nationally, then go for it. Top 10% ALMOST always get what they want.
To answer your other question, I found that my ROTC experience greatly helped me in my med school app. It showed that I had leadership experience, was capable of handling stress, and helped set me apart from the crowd a little bit. Keep in mind though, ROTC was not my only activity. I was a leader in MANY other things as well as an EMT-IV and had tons of clinical experience, and a 3.98 GPA. If you are capable of handling the stress and think you can be one of the top, ROTC is a great way to go, but know, you will be in the military and you need to understand what that entails.