D Mac IFS is very new. Ramp up is as follows: FY02 350, FY03 800, FY04 and beyond..all USN, MC and CG.
IFS intent is to measure SNP aeronautical adaptabliity, enthusiasm, motivation and desire to fly.
You fly with a FAR part 141 school. Programs will may differ from place to place; but here's the intent and reason.
You are approved to receive civilian flight training. You make the appointments, you get ready to fly, you drive your own sylabus with a civilian flight instructor. The Navy removes the financial barrier to flying. Those who complete the requirements by the deadline are motivated to fly; those who don't, aren't. Saving the Nav $$. Civilian flight instructors and Cessnas are cheaper than senior LT pilots and T-34's.
Now what does it give you that API doesn't? You don't get any "houses get bigger, houses get smaller" monkey skills in API. Benefit: you get some prior exposure to FR&R, Comms, Basic Aero, etc: Oh yeah, you get to go flying for free.
BTW, no extra points for finishing IFS quickly.