most of these have already been stated:
a) you can have exactly the rifle you want...
b) you will end up spending less than a fully assembled rifle, though not a huge ammount...
c) you dont need to have a wad of cash all at once because you can pick up parts as you go...
d) you will understand how the rifle goes together, how it works, and have a better understanding of how to fix it if it malfunctions...
e) you can take pride in the fact that YOU built it...
f) since you were the one who built it, you have the opportunity to check each and every part over before assembly to ensure that there are no defects... plus, this way you know exactly what parts went into the rifle, and you know it was assembled correctly (if you followed the instructions)...
g) you can always ask for help here if you run into any snags... ive been all over the net and i can honestly say the this is one of the best communities i belong too... everyone wants to help, because we were all in your shoes at one time...
h) you can assemble most of the rifle without any specialized tools (except for one of those monster barrel/car stock wrenches)... if you do buy specific tools (roll pin punches, the combination wrench, torque wrench, vice blocks, headspacing gauges, etc), the best part is once you have em, you can build any number of ARs!!! which is a good thing, because believe me, once you build one, its all over for you... this isnt a hobby, its like some kind of disease!!!
its too late for the rest of us, but you can still SAVE YOURSELF NOW BEFORE ITS TOO LATE!!!!