Good points to consider Mike.
The break-down that HNeedle gave is almost right. It is 55% GPA, 35% MPA, and 10% PEA. Now do some crunching on this. In the OPA, the difference between a 2.0 and a 4.0 PEA is only 0.2 points. BUT, MOST people fall between a 2.5 and a 3.5, which is only 0.1 points. A star athlete that might be around a 3.7 (VERY rare) and a top academic that might happen to have a 2.3 will see 0.14 difference on the OPA.
The difference for a 2.0 MPA and a 4.0 MPA will be 0.7 and 1.4 in the OPA, or only 0.7 points. MPAs will mostly fall between a 2.7 and a 3.2 for 90% of people. This translates to a 0.175 point difference between a 3.2 and a 2.7. The outliers, with the new system that will affect newcomers, will be a high of maybe 3.5 and a low of 2.2 or so. The gap is very small.
Now the GPA is a huge game changer. Between a 2.0 and a 4.0 GPA, that is 1.1 and 2.2 points in the OPA or 1.1 points overall. The typical GPA does not fall in a little gap. 90% will fall between 2.1 and 3.8. This gap is therefore about 0.94 points in the OPA. 0.94 means that the effect of GPA is realistically 10 times more than the PEA and 5 times more than the MPA.
Some examples:
3.1 PEA, 3.1 MPA, 3.3 GPA --> 3.21 OPA. A typical supts pin person.
3.3 PEA, 2.6 MPA, 3.9 GPA --> 3.385 OPA. A friend of mine who is only concerned with going to med school and doesn't care about the military
2.3 PEA, 3.1 MPA, 2.2 GPA --> 2.525 OPA. Another friend of mine who is awesome at the military but struggles elsewhere.
2.4 PEA, 3.0 MPA, 3.6 GPA --> 3.27 OPA. A typical geek that isn't so athletic.
2.4 PEA, 3.1 MPA, 3.4 GPA --> 3.20 OPA. Supts pin above but not so athletic with one more GPA point. (ONLY 0.01 difference in OPA!!! 0.6 drop in PEA but 0.1 increase in GPA)
3.4 PEA, 3.1 MPA, 2.3 GPA --> 2.69 OPA. An incredible athlete and top military person but struggles academically.
2.47 PEA, 3.084 MPA, 3.8 GPA --> 3.47 OPA. Myself.
See how important the GPA is in this equation?! A high GPA can topple most high achievement anywhere else. Despite being in the bottom of my class militarily and near bottom physically (about a 2.3 PEA) as a freshman, I still was ranked about 70 overall at the time because my GPA was a 3.65. The GPA is the most important factor in your class rank.
Now, for how that works for the AFSC. February of 2 dig year our BOM (Board order of merit) will be determined. This is done by taking a person's class rank based on the OPA and changing it based on their Form 94. The Form 94 documents all probations, all jobs held, and 5 bullets that you will write and proof with your AOC that documents additional achievements (like scholarships, leadership experiences, etc.). Using this, a board of 4 or 5 colonels will secretly take everyone's Form 94s and alter their class rank from what it is. Usually, it won't change more than + or - 5 places unless a Form 94 isn't turned in or is blank (then that could drop a person to the bottom, regardless of OPA). So, a person's rank when considering their AFSC will be within probably 5 ranks of where their OPA says they are.
As a cadet, that means that the GPA is the most significant factor in getting their AFSC. While we have over 500 pilot slots so that most anyone qualified will get pilot, everything else will then be competitive, usually based on the BOM for all qualified candidates. What happens, is people input their preferences for AFSC and rank them. Then, they take everyone's top pick and rank them against each other in those fields by the BOM. So, say intel has 23 slots and 55 people put it as their top pick. People will be able to see how many they are competing against and where they fall in that list. So, the top 23 BOMs in the intel list will get intel. Everyone else will then be dropped into their second choice lists and so on for up to 6 spots non-rated and 3 rated (rated are pilot, nav, and air battle manager). So, if you are ranked in the BOM in the top 10%, most will get their top pick.
Some programs that are a bit different are CRO/STO and ENJJPT. CRO/STO has its own boards and side programs that change things up, but two equivalent people will ultimately be compared by the OPA. ENJJPT is a subset of pilot. So, of the 520 or so pilot, 200 may apply for ENJJPT. The ENJJPT boards will then meet in June and rank everyone. While they have some quotas for people from the WoB, soaring IPs, and flying team members, most everyone will be ranked by the BOM. Since they have about 30 of those slots a year, the 20 slots that aren't reserved for any groups are then decided by the BOM rank order of those qualified. The higher one's BOM, the more likely to get the slot. I know a lot on this one because this is my goal!
To give some idea of where the OPAs sit. My OPA of 3.47 puts me at 47 of 1045 people in 2010. A 2.5 is about 7-800, a 3.27 is about 80, and a 2.7 is about 550.
Now, why would it matter after USAFA and jobs? Well, when the first promotion boards for 1st LT come up, they will have all 1000 of us that will be promoted (ignoring ROTC and OTS for this consideration). But not everyone promotes at the same exact time right? It's done in monthly increments. Many will have great achievements from their 2 years that will help them promote early. What happen to the 400 or so that might be in some part of pilot training or just entered their units. Most, say 300, won't have significant achievement to set them apart. But that many can't be promoted at once. So, what will they use to decide? How about their USAFA class rank.
Someone graduating in the top 10% vs the 50th percentile or the bottom quarter is likely to promote first. A couple months right? No big deal. Repeat for captain. Hum, we have 30 captains that are pretty similar, but they're academy grads, let's promote the top grads first. Then the same happens at the LtCol boards. This means that the top grads could be a couple years ahead of their fellow grads which translates to higher pay and maybe better retirement, no?
Lots of things to consider. But when it comes down to it, GPA makes a huge difference for cadets.