SAML support between the core Google Apps (mail, docs, calendar) are well supported. We have observed some issues with some of the applications in the Google Apps Marketplace. While they share the same Google Apps user name, the SAML support is not extended to those apps and as a result, the user is still prompted for credentials.
Be aware that some apps from Google Market place may not be as tightly integrated as they should be with Google Apps in terms of SSO. Let Okta know about any issues and Okta support will help you solve any problems that may arise.
Users will be sent to the Okta login flow in most cases, but we have encountered some problems on Android devices where the language setting is not en_US. The current work around is to set the device language preference to en_US, authenticate, then change the language back to the user's preference.