I spent 9 years in an Army uniform and everyday of those days I had a multi-tool on my belt and a folder clipped in my pocket. I would say that 99% of everyone else I knew used the same. If I was on duty again, I would probably replace the folder with an Izula with some scales.
Some people like fixed blades in the field and when deployed, but in reality 99.99% of the situations that call for a blade can be solved with those two items and in my opinion the extra weight of carrying a fixed blade never was justified by the utility it could possible provide. Most of the time those that had them never used them and simply had the pleasure of carrying more weight. In my entire combat arms career (including one Iraq, and two Korea assignments), I never once felt the need for a larger fixed blade or was in a situation where I wished I had one. They issue so much crap to carry in today's forces, you are not going to want to carry anything more than you absolutely have to.
Spend the money on a quality multitool, but don't spend a lot on a folder. If you are spending more than $60-70 on one you are probably wasting your money. It is going to get beat to hell and chances are you will loose at least one or two over the course of several years especially if you are combat arms. As far as folders go, my personal favorite for military use these days is the Salt line by Spyderco. They are very high quality, relatively cheap, razor sharp, the steel is completely rust-proof (ie it will NEVER rust).
Lastly don't buy anything until you get to your unit. Some units don't care and don't regulate knives, but some do and there is no way to predict it. Most units did not care when I was in, but the Army has gotten a lot more anal about that sort of stuff since I left. So you might be rather disappointed if you go out and drop $100+ on a new knife only to later find out it gets to stay in your room everyday.
