The War Powers Act on its own has not been a significant restriction on the ability of the US president to wage war. It is one tool that Congress can use to pressure a president, but it has never, on its own, been used to prevent a president from acting as he wished.
A major reason for this is that the War Powers Act has never been formally declared constitutional. No president has ever acknowledged that the act actually has any force. This keeps the act from being a very effective restriction on the president's ability to wage war.
However, this is not to say that the act is meaningless. It can be used as an effective rhetorical tool against the president. It can force him (or someday her) to justify his actions to the American people. It is public opinion that ultimately ends up being the most effective check on the president's ability to wage an undeclared war.