Usability 
Opera Software's mobile app store supports Android, BlackBerry, Java, Symbian and legacy Palm OS and Windows Mobile devices and includes both free and paid apps. When I tried it the only payment option available was by credit card.
Opera is promoting the Mobile Store in Opera Mobile and Opera Mini, where it's linked to from the browser's Speed Dial launch screen. However the store works in virtually any mobile or desktop browser.
I tried the Opera Mobile Store in Opera Mini on a Motorola i1 (Android), Nokia N8 (Symbian^3) and Motorola Rambler WX400 (a low-end Java feature phone). The webapp correctly identified the i1 and N8. With the WX400, I had to pick a phone manufacturer and model. The WX400 was listed but took a lot of scrolling to find in a long alphabetical list of models.
App discovery in the Mobile Store leaves a lot to be desired. The site search often returns completely irrelevant results. For example, a search for "file manager" brought up several personal financial apps, a web browser, two password managers and a music player, but no file managers. Searches for specific apps by name worked better.
You can also browse the catalog, which is divided into categories like Tools, Internet and Calculators and also has Top Paid, Top Free and Featured groupings. The categories are quite broad, all games are lumped into a single category, for example. A number of apps are miscategorized with aberrations like the Calculators category which lists several games but no actual calculators on the first page of results.