Last week it was announced that iAds - ads within iPhone and iPad apps - were finally coming to Europe, over 6 months after the initial announcement of iAds in the US.

iAds were initially announced in a blaze of publicity by Steve Jobs in April, and were immediately predicted by many to be likely to revolutionise mobile advertising by offering previously unseen engagement and creativity.

However, 6 months on, only about 15 iAds have been run, as far as I can tell, for companies including the Nissan Leaf, Libery Mutual, Sonos, Dove, McDonalds, BMW & Geiko. (You can see links to many of these on my blog). This is fewer than was expected, and it also points to the potential and perceived problems with the format.

First, the ads can only run on apps within Apple's operating system. Yes, the Apple OS also caters for iPod Touches and iPads, but Apple still only had a 17% share of the mobile operating system market in the third quarter of 2010, according to Gartner. While Apple users are attractive to advertisers, it's restrictive to use a format that can be seen by fewer than one in five.

The second factor is the cost. Apple has reportedly set very high entry costs for advertisers - some reports say seven figures - and payment based on exposure rather than results. This would be a huge amount in most media, let alone a comparatively untried one.

Third, Apple has been very protective of the format in terms of creative. It's understandable for them to be very careful to ensure high-quality creatives for such a new format, but agencies don't have as many restrictions from a single supplier in other media.

Finally, there is currently no showcase to show off the ads. It's currently really hard to see how iAds look, apart from the few that have been put onto YouTube or covered in marketing blogs. It would be really useful for agencies to have an 'iAd' app that we could download to see real examples of live ads, or even an official YouTube channel.

It's also important to consider that there are lots of other ways to put ads onto iPhones and other mobile devices. AdMob (owned by Google) has some very engaging formats, and other companies have other offerings, like ads as interstitials within mobile games.

The iAd announcement generated a huge amount of buzz for Apple and mobile advertising, but as the Wall Street Journal has reported other providers are saying that Apple has given them a boost.