Emarketer has a good overview of current research supporting online advertising:

  • The audience is more affluent
  • It’s effective in changing users’s perceptions of brands.

Robert Losch at Marketing Fix adds

  • It’s the best way to reach the work/school audience
  • Consumers are using it to find product information

DoubleClick claims that by increasing the amount of Internet advertising in your media mix, you can increase frequency and brand awareness.

I would add that it’s the best or even the only way to reach people who have begun to limit their consumption of commercial media: TV nonviewers, DVR users, or NPR listeners. Many of the people are as influential as they are difficult to reach.