I am going to stay in for the whole career. I have 12 years in (roughly) right now, and my commitment takes me out to 16, so I am definitely going to stay in. My criteria is, if I am still having fun, then I am going to keep doing it. I personally like putting on my uniform (whether it is the flightsuit or khakis) each day, and serving my country, especially during a period such as this where we are needed to do our part. $$ aren't everything (and no offense Frumby), but flying from point A to B and watching the fuel comsumption and making sure I am on time is not something I relish, even if the bucks are so much better. For me, icing on the cake would be to make it all the way to having my own squadron and being a CO, then I could get out and say I did it all. But for now, I am just trying to survive my JO tour, and still have a good time.
As for test pilots, yes, you can put in for that, we have had two NFOs get picked up from my command in the last year, and two of the pilots applied but didn't get in. That is not a route I am interested in, I would like to become a P3 instructor pilot and go to the FRS and teach, or failing that go to the VT as a T34 IP. I think teaching others to fly would be a blast.
Reserve P3s are not going to be a factor for me, I will do my time and retire, and go on and do something else for a living.
And yes, I have heard that the P3 sets you up nicely for getting into the airlines, the hours don't hurt that is for sure. But in a market with what 6,000 or so furloughed pilots, I don't think the majors are hiring right now (expect those isolated cases like Southwest and Jetblue). If airlines are your eventual goal, then you might think about the E6 TACAMO and fly the 707, I am sure travislikes would have some input there.