They're overrated when both sides are using them, as it ends up largely being an inflationary effect that cancels out your opponent's potential advantage from using them. When it is entirely lopsided, however, they provide very real benefits to the winning side. This effect is observed in just about every aspect of a campaign in terms of field ops or media: even with freaking yard signs.
Reminds me when this site was so happy (or upset, depending on who was talking) that Corbett wasn't airing many ads in Pennsylvania in early 2014 when Wolf was monopolizing the airwaves.
