The downside to running all those beefy components is that the battery in these mobile monsters can't power things for very long. The M17x's 9-cell battery was just about equal with most of the field, although the Maingear and ASUS G74SX systems wrecked the curve.
Bear in mind how widely battery life can fluctuate. You can extend a battery's charge by opting for more efficient power profiles, not to mention simply manually adjusting things like screen brightness; and of course, less demanding tasks such as word processing or Web browsing won't suck the life out of the battery as fast. We test these systems essentially as-is, so there's room for improvement on these times.