I've built two now, both on (the now defunct) Sabre lowers rollmarked with a local gun shop's logo.
The first is my "hall clearer", with a RRA middy flat-top upper, RRA BCG, Magpul MOE forend, GG&G flip rear sight, Doublestar LPK and a VLTOR EMOD stock. Also have a Streamlight Polytack for a weapon light. This is a decent shooter, and I used it as my 3-gun rifle for a couple years before I built my second.
Second I built up from parts - including the upper. Upper is built around a BCM upper, BCM bolt, and a Nordic 18" rifle gas Barrel. Muzzle brake is a DPMS Miculek, and it has a Troy Alpha rail (I'll never buy anything else from Troy). Lower wears an A2 stock and a Doublestar LPK with a Chip McCormick 3.5lb flat trigger. Optic is a Burris MTAC 1-4 in a Burris PEPR mount. This is a fast, fast rifle for 3-gun. At 1X, the MTAC acts like a red dot, but it's got enough precision at 4X to reach out a ways. It's sub MOA with a different optic and 69 grain sierra match kings.
Building is not hard at all, and very rewarding. I've helped new AR owners put together about 8 of them now, and they all go together just fine. If it's going to be something like a patrol rifle, buy a Colt, DD, or a LaRue (if you can swing it). If it's a personal-use or competition rifle, building or buying are both worthwhile options. Building allows you to get exactly what you want, but you probably won't save much money in doing so.