Last August the FCC published a report on US ISP's "actual vs. advertised" performance (see "FCC: "Most ISPs delivered actual download speeds within 20% of advertised speeds" - here). A major exception to the generally good results (80+%) was the performance of Cablevision (see chart below).
It seems that public monitoring helps. In a post to the FCC blog, Joel Gurin (pictured), Chief, Consumer & Governmental Affairs Bureau, reports that "As we had hoped, our [August] results quickly began informing the marketplace. Our findings were reported not only on the news and on the websites of consumer groups, but also in television commercials, radio ads, and press releases from Internet service providers themselves .. We are pleased to note that the performance of one company—Cablevision—markedly improved from earlier this year. .. During October 2011, the most recent month for which data is available, subscribers to Cablevision’s 15 Mbps service were receiving average download speeds during peaks hours at over 90% of the advertised speed"
Measuring Broadband America,
See "Broadband Speed: FCC Data Is Improving the Market" - here.
In the meantime, Cablevision filled a lawsuit against Verizon - "for allegedly making false claims in advertisements for its FiOS high-speed Internet service .. Verizon's ads [see below] misleadingly presented Federal Communications Commission data to suggest Cablevision service runs exceptionally slow. The FCC figures actually show Cablevision Internet running near or above advertised speeds even during peak usage hours" (see WSJ article "Cablevision Sues Verizon Over High-Speed Internet Ads" - here).