USERRA
It is a great deal. I myself took mil leave for 3 years, bonus too, and I came back without any problems, seniority, vacation and contributions, as if I had never left. Naturally, my chief pilot was pissed off, but there was nothing he could do about it, federal law, baby! I was off probation before I left.
I'm too far away from even reaching sanctuary, so being able to retire active duty wise is not in the cards for me. For me, it allowed me to do something new, simplify my work life, be home more, and make a lot more than B scale pay, and when I returned I had my pick of bases and aircraft, plus a serious bump in pay. Still, like anything else there are some fallbacks, however everybody is different and looking for diverse slots, there are even some staff puke jobs that go begging, certainly with enough reason, but I would be willing to do that if it meant securing a bird in hand retirement that is indexed for inflation. Most importantly, I would try for an AGR job and not return to ADuty with the Air Force.
As for the 5 year max, it is true there are exceptions, just make certain yours meets one of them, and don't expect to pencil whip the exception because you may not have a strong case to do so. I think a letter from your commander would be sufficient to do that, but there might be an issue that it is has to be something along the lines of it being required by the SECDEF, and with the war on terrorism seemingly being here for the long run, who knows, at the end of your 5 years you might be able to extend your stay, it certainly cannot hurt to try and at least get to sanctuary, which is like tenure, and weigh your options. Naturally, if you push the limits beyond 5 years your company is going to look closely at your rationale for doing so.
As somebody stated earlier if you were forloughed and thenwent on Active Duty your 5 year clock does not start until they call you back.
As for ALPA I had to give up my membership at the time, but who knows now with a lot more guys on ADuty they might have changed their rules on that, but I would think that if you had loss of licenses or medical insurance, that you would not be covered if you became injured on active duty, there might be a war/military exception to their insurance coverage.
Good luck with your tours!
PS Make certain you have your company 401K set up for max contributions before you jump too. I think that if you do set up an IRA with the govt plan, that when you return to your company, that there might be some limitations on your actual make up contributions wiht your ocmpany.