Like 'em or not, the mainstream nightly broadcasts still draw around 30 million viewers a night... so they're still relevant in American life.
As for balance, talent and the ability to effectively use 22 minutes per show..I'd say on merit they fare well... though like any form of information/entertainment they could always do better.
Still, I'd take Brian Williams over the rest any day.
Correspondents by and large (men and women like Tim Russert, Andrea Mitchell et al... and anchors like Brian, Lester Holt, Matt Lauer et al may not have all the answers but they at the very least they do reach out to millions of Americans and try to provide context, perspective and hopefully a snapshot of our nation.
Best of all, they're going to get better because viewers are becoming more sophisticated and with so many choices along the viewing landscape broadcast networks had better improve or they'll face extinction.