Overall it helps, especially if the Army is sending you and even more so if they are sending you in order to go back and teach at West Point. Many graduate admissions offices place a high value on the maturity and work ethic of academy graduates.
However, the downside is that West Point graduates often have lower GPA's than they would have from a civilian college. Cadets have a larger workload, both academically and extracurricular, than their civilian counterparts. Also, the West Point academic program is rigorous and requires more demanding coursework outside of the major than most universities require. Many, but not all, graduate schools will account for this.