I'm actually in the process of doing this right now... It is unfortunate that you have to give up one weekend per month, but then again, in my case, I am obligated to serve because I still owe another year of reserve time to finish paying back my ROTC scholarship.
Basically, from what I understand, you are in a non-deployable status (which is why I am expediting my transfer from the Engineer Branch to Medical Service Corps)... Hopefully we don't decide that Mr. Hussein should take an early involuntary retirement in the next 2 months, or I might get called up.
Also, just fyi, unless you have prior officer experience, you will probably have to go to officer basic course (ask your recruiter to be sure)... plus, one thing I found out is that prior officers actually get demoted when they join MSC... so I'm enjoying my last drills as an O-2 before I become a butter bar again.
AFAIK, you'll probably join a MSC unit and be doing physicals/paperwork on your weekend drills. That's what they have me slated for.
-ttac