What happens during the summer varies with what opportunities are available. In my case (I joined after my first year so it was only 3 summers) I did the first half of basic training the first summer, the second half of basic training plus french language training the second summer and spent the third summer up in Yellowknife and Whitehorse doing "on the job training" or OJT and working at a cadet camp. Most students do some sort of OJT during their summers after basic. This usually consists of working at a base Medical Inspection Room or "MIR" which is like a military doctor's office. This is a good opportunity to get some experience with the way basic health care is delivered in the Canadian Forces and exposes you to a lot of the paper work that we deal with.
As far as pay goes, you can look at the pay scales here DGCB - Regular Force and Class C Officer Rates - 2012
You would fall under pay level A as an ROTP candidate. Basically, while in school you would start at about $18400/year (plus your tuition and books are paid for). Once you graduate and become an officer you jump to $57500/year. After roughly 10 years in you are looking at $78500/year.
Yes it is competitive but you are accepted before your training and not afterwards. So basically if you are accepted for ROTP you are already a member of the CF, the training comes after that. Basic training is tough but it is not about competition it is about teamwork, learning and leadership.
Once you graduate you can expect to be posted. You are able to submit preferences as to where you would like to go but in the end the military will move you to where they need you. For nurses this is usually in Edmonton, Ottawa, Valcartier or Halifax, with some nurses ending up in Vancouver and Petawawa as well. There are some postings in some other places but they are few. For what its worth, I have been posted to all the places I have listed as my top preferences, but other's mileage will vary greatly.
The work we do also varies greatly. The CF doesn't have military hospitals anymore so that isn't an option. Once you are an RN you will be placed in a civilian hospital for a period of time (around 6 months now I believe) to gain experience in that setting. After that the road splits into many directions. You can apply for specialty training (Critical Care, OR or Mental Health) or you can work on base in the MIR. There are also many administrative positions that nurses frequently fill in areas such as training and operations.
Finally...you can't join the military to get rich, but as you can see from above, the pay isn't bad either. Getting paid to go to school plus having your tuition and books paid for is a huge bonus. To make it simple, right out of school you earn the equivalent of roughly $27/hr (of course you are on salary so its not really "per hour" but you get the idea) and this goes up every year. Plus paid vacation time (20-25 days a year) plus full medical and dental.
Obviously money is important but I'll reiterate, this isn't a normal job. We have periods of quiet and we have periods of craziness. We are sent to places that few would want to go and are placed in situations that many would never want to be in. We do this without questioning and without the option of saying no.
Food for thought.
Keep the questions coming...I obviously love my job and could talk about it for hours.