The use case is to have an application store data on the client side when offline. Is it advisable to use the Web SQL Database (which Chrome and Safari support, not FF though), or wait for the browsers to implement the Indexed Database API?
9 months after this question was posed and the Web SQL Database is "..no longer in active maintenance and the Web Applications Working Group does not intend to maintain it further": http://en.wikipedia.org/wiki/Web_SQL_Database.
If you are developing a solution for release next year (especially mobile) then IndexedDB is the better option. If you need coverage now (excluding Firefox), then you can use Web SQL Database or serializing JSON objects into LocalStorage.