I really like my AR's. (Except I lost a lot of 17 Remington brass shooting coyotes before I bought a brass catcher!).
To answer the build it or buy it question... I'd say build it. Buy a good quality wrench, vise blocks, and a DVD. Then shop for parts.
If you must by it, watch the "Bargain Bin" at CMMG. I have bought a couple uppers and one complete rifle from them, that were all cheaper than I could build them.
Got my last two antelope with a DTech upper in 25 WSSM, my wife got her speed goat with her 6.5 Grendel.
Looking at building a 20 Practical, or a 6 DTI next.
AR's are like eating chips...you can't stop at one!