The modern banner ad is generally, in a word, annoying.
There have been some interesting developments in the past few years, primarily the use of banners that require user interaction. These tend to be in the form of ads promising free stuff in return for “winning” a tiny game. The attempt here is to engage the viewer, and I must admit that it occasionally works on me – I will sit and play a little banner game for a minute. However, I immediately close any page that the ad then directs me to, so I don’t know how successful that strategy is.
From a design standpoint, these ads tend to be quite ugly – they are dominated by garishly bright colors, and often include text of illustrative elements which are simply of poor quality.
The most visually pleasing ads, rare though they are, are the ads that have been designed specifically for the page they are on. These tend to be found on webcomics, where some artists will draw up an ad so that it doesn’t clash with the site, though it still looks very much like an ad, so that it is not confused with the site itself.
However, from the standpoint of a web surfer who doesn’t want to be bothered by flashing graphics, ugly colors, or, God forbid, ads with audio, the most successful ads are the text-based Google ads that some sites use. They are clearly labeled as ads, but are skinned using CSS to match the scheme of the page. These ads actually present links to content that might interest the user, judging by the content of the page they are currently on. This is a far superior advertising technique to simply shouting a random message at anyone who happens to come by the page.
So where are ads going? The trend of interactive ads is at very least intriguing, so perhaps it will continue. However, I hope that the quality of these ads will increase. The Google ads illustrate a trend towards matching ads to markets that would actually be interested in them which would be good to see emulated throughout the web. As a designer, I hope to see (but not make) ads that are more visually refined, silent, elegant, but still easily identifiable as advertising. (As the folks at Google say, “don’t be evil.”)