Powerybase is reporting a surprisingly drastic drop in sales of their previously popular and steadily-selling ToDo reminder app NotifiyMe following the release of iTunes 9.

Their data shows a 20% to 30% hourly drop in sales following the update, which they say has been steadily growing as more people adopt iTunes 9.

Powerybase blames the drop in sales on “bad interface design” in their opinion in the new App Store, ranging from navigation buttons in the new toolbar, the disappearance of sub-categories for games, and numerous other not-so-subtle differences that result in an overall unpleasant and difficult browsing experience.

Some countries don’t even have categories for apps, only showing links to the App Store home page, according to the developer.

Powerybase concludes that Apple has made many of these changes in an attempt to force developers to rely on using keywords to make their app visible instead of long names and descriptions, as well as getting icons that visually stand out.

They write more in their blog: http://powerybase.com/blog/post/2009/09/11/iTunes-9-interface-and-a-2025-sales-drop.aspx