It's not my area of expertise, but these seem like pretty good ratings for a half-hour infomercial in prime time.
The combined overall household rating for Senator Barack Obama's Wednesday night infomercial, in the top 56 local television markets where Nielsen maintains electronic TV meters, was 21.7.
Obama's simulcast is the first to be aired by a presidential candidate since Ross Perot ran a political telecast on Election Day in 1996. That program was watched by 16.8% of all households nationwide. [...]
In comparison, the final debate between the two presidential candidates received a 38.3 household rating in the top 56 local TV markets. The candidates' first debate on September 26 received a 34.7 household rating in the top 55 markets; their second debate, on October 7, received a 42.0 household rating in those markets.
The New York Times added that the commercial "was particularly high-rated in several battleground states," including Pennsylvania, Florida, and North Carolina.