LOL I wouldnt go that far. It was a fairly bloody affair for both sides and while Custer did distinguish himself there he hardly did it all by himself.
Through out the course of wars, you can pick out the singular point where the tide of the war was changed. If that singular act didn't happen, the war would have been completely different.
Now, I'm not talking about the first day of Cavalry fighting at Gettysburgh, I'm talking about the very last day. Had Jebb Stuart made it through and hit the Union lines, the confederacy would have WON. And since this was the turning point of the war, if they won Gettysburgh, they would have won the rest. Hence, Custer saved the union.
(I can actually point out the singular act that saved the Revolutionary War too).
Custer gets overlooked a lot because his ego got way too big when he was fighting the Indians.
It's like that saying: Build a thousand bridges and no one will ever remember your name. But if you ever suck...