I separated from AD at just under 17 years in 2012 to take an ART flying command position. At the time it made the most sense - I planned on an AF career and wanted to keep flying. I felt the AF Reserve would give me the best opportunity. Two years later I took an AGR staff job. I felt it would help career plans and allow me to "bank" 20 years of AD time when/if I wanted to retire completely. Shortly after that I started to consider the airlines more so put my apps out. Got a call from a legacy and got a job offer (after 2 years of non-flying). Separated from my AGR tour at ... wait for it ... 19 years total AD time.
However, the only reason I was comfortable in doing so is because I was able to stay on as an IMA/part timer in my current position, with a healthy man day budget, and a strong likelihood of a return to AD on mil leave for a period of time in the near future.
Those mil/airline guys who I tell the story to agree it was a smart move. Hopefully it works out. I tell you this mainly because every situation is different. Research as best you can but none of us can safely see the future. Determine your own personal level of risk you can afford to take and never shut any doors.
The previous post is correct about treating high-AD time reservists differently when it comes to AD retirements. I've heard the same data about the Navy as previously posted - they generally frown on it. On the other hand, historically the AF Reserve has been very good about it. 20 years AD is 20 years, they won't stand in the way of an assignment to prevent having to pay for that retirement.