The beauty of Guard and Reserves is that you pick where you want to work. Once hired by a unit, your two years (or so) of training is completely laid out for you with a guarantee to work where you got hired. So no matter where you rank in UPT and such, you are going to work for your unit that hired you and fly the plane they fly provided you pass and get through training. It takes a lot of the pressure off and you can just concentrate on learning, flying, and enjoying the training.
Figure out what plane and mission you want to be a part of. Find out what the lifestyle is like for the planes you are interested in. Like everyone else has said, what the requirements are to keep current, how much full time work they can offer and so on. I'm a Charleston Reservist, and for the last 5 years I've been here I've done nothing by fly the 17 and have had plenty of paychecks. I have no other job right now. Charleston is rich with missions (just not money right now. A WHOLE separate topic. LOL!) and therefore you can always find lines to fly. If you don't want to be full time and just work when you want to, you can. If you want full time work we have that as well in rotations. What I'm getting at is not every C-17 unit is like us. You'll find many different situations out there. You will have different experiences at different places with each plane so do your research.