PocketGamer.biz has an interesting interview up with Pangea, makers of the iPhone physics puzzle game Enigmo, and they say that so far, their experience with in-app purchases has been pretty lukewarm. The software itself has sold over a million copies, but only a very small number of those customers are going for the in-app purchases -- about 25 per day, according to Pangea's Brian Greenstone. 25 per day may not be bad for a smaller developer, but Greenstone says he "was expecting hundreds."
Why aren't the microtransactions doing so hot? It's going to take time, first of all -- customers need to learn the value of picking up content piece by piece, and developers need to learn exactly how to price and plan the stuff out. On an established download service like Xbox Live, add-ons and content packs have become de rigueur, though some of those battles are still being fought. But on the App Store, not only are prices still up in the air, but in-app purchases are hard to find, and they're all over the map in terms of value versus investment.
Greenstone seems to believe what I'd tend to agree with: that after all of the dust has settled, customers will pay for content that's worth paying for -- he's just given up on worrying about pricing, and is focused on delivering content that's worth whatever he wants to charge. Eventually, both devs and customers will settle on a fair price, and when that happens, I'm sure we'll see some major in-app purchase sales.