I assume you are considering a S&W M&P-15 Sport. That rifle is building a pretty good reputation for itself. They run about $650-699 from what I've seen.
However, consider this. For about the same money (less if you build your own lower or if they are on sale), you can get a Palmetto State Armory rifle that includes a chrome-lined barrel, forward assist, and dust cover (as well as other plusses like a hammer-forged barrel). Seems like a no-brainer to me.