America's Army was actually a pretty respectable FPS at the time and had it's own niche in the gaming community. There was a huge hole in the market in the early 2000s for anything realistic modern warfare related; CounterStrike was pretty much it. Everything else was stuck in the WW2 fad (CoD 1/2, MoH, Red Orchestra, BF1942) or pure fantasy (Doom/Quake/Tribes/Halo etc.)
I miss Tribes
Americas Army was the most intuitive game I ever played.