Brand building is a long-term activity while lead generation is short-term.
Brand building yields few visible results in the short term, but carried out consistently over a period of years, provides an outstanding support for lead generation in the short-term.
TV ad campaigns can effectively build brands, for sure. This includes the high-tech industry or any industry.
* The Apple campaign pitting the cool Apple person vs. the Microsoft geek decimated their image and built Apple's to the hilt - it was not about selling product but building an image.
* The Intel campaigns with their distinctive "bam bam bam bam" sound, and the employees behind it, built an image for the company as a leader. Of course ads work.
So the question is not whether ad campaigns work but whether they work FOR YOUR PURPOSE. And whether they yield sufficient return on investment.
If the sales campaign (you say lead generation so it makes me think sales) is oriented toward a business customer, TV ads probably are too large an investment to yield a short-term result. Word of mouth and trade shows would be a more cost-effective route.
If however you are aiming to the average person like myself, and you seek to reach a large audience, then TV ads start to make more sense as a way to build a base for later sales efforts. This is particularly true if you're trying to sell expensive product - TV ads make it seem that the product is well-made and well-funded.