I have about 15 iOS apps on iTunes, and sell/distribute two Mac apps from my website. The only plusses I see from iTunes are ease of distribution and checkout. The downside is a massive loss of control over versions, updates, demos, and bug fixes. Having to wait 7-10 days for Apple to review and approve every update is stressful. There's no worse situation to be in than having an urgent bug fix stalled by the Apple review cycle. On the Mac side, I can push updates instantly.

Now, one big plus of the App Store is exposure, but in reality, very few developers benefit from it. Being featured or noticed on the App Store can make or break apps. I can compare different pairs of my similar apps, and the only thing separating the successful ones from the duds is that the successful ones got featured at some point. If your app is not featured, it will not get exposure and won't be noticed on a large scale, thus negating any perceived benefit of being on the App Store.

I think I read somewhere that less than 10% of devs make any reasonable money on iTunes, and of those, many do it with large external marketing budgets. That means that those of us who have benefitted from Apple features end up being a small fraction. If I had to guess, it's probably 2-5%. So I think it's reasonable to estimate that 95-98% of devs are not really seeing any sales/exposure benefit from being on iTunes. If the Mac App store is smaller, that may be different. But if there's a gold rush and it blows up, I think most developers will continue to get lost.