From a recent whitepaper by Bunnyfoot Universality:

Banner adverts are still the most popular advertising format, comprising over 40% of all online ad spend. This equates to £120m spent on banners alone in 2003. Banners have a broad definition as a graphical web advertising unit. Its typical measurements
are 468x60 however recently the IAB believed there to be over 10,000 different formats and have introduced a Universal Ad Package in 2004, consisting of six standard shapes and sizes. We included in our study all embedded graphical units, from standard banners to skyscrapers to buttons.

A summary of the main findings:
• Users look directly at banners for nearly a quarter of their often within the first 5 seconds of their exposure to the page
• People are able to recall an advert they had seen without prompting
• Over a third can recognise an advert they have seen, a more valuable for credibility and brand building.
• A colourful presence made more of a lasting impression
• Dynamic adverts provided better recollection in Bunnyfoot testing.
• Location of advert on the page determined its level of success
• Users were mostly neutral in their attitudes towards banner
• Negative comments were few in number but were exhibited tried to encourage click-throughs via deceptive formats.