Derivante

GAPI 1.3 released this past month. First, read the Google Analytics Data API Reference and then read up on dimensions, metrics, and valid combinations of the two.  The quotas apply to a single web property, so each analytics profile (site1, site2, etc) are subject to their own individual quotas.   There is no per user account limit for accessing the API.    If you plan on making any kind of on demand application I suggest you query in bulk and cache the results locally (analytics is not real time anyways).

API Features

• Supports CURL and fopen HTTP access methods, with autodetection
• PHP arrays for Google Analytics metrics and dimensions
• Account data object mapping - get methods for parameters
• Report data object mapping - get methods for metrics and parameters
• Easy filtering, use a GAPI query language for Google Analytics filters
• Full PHP5 Object Oriented code, ready for use in your PHP application

Google Quotas

• The quota applies to a single web property
• 10,000 requests per 24 hours
• 100 requests in any given 10-second period
• A query is also limited to pagination limits of 10,000 entries per feed, with a default response of 1,000 entries

While the broad set of data they have is extremely useful to pull down to do your own processing on, I find the custom visitor segments with the API a lot more interesting.    If you have a user based site with registration and collect information on your users, you can pull this data back out to determine a lot of information about your user.   A lot of tools are already built into Analytics for reporting but the ability to mash this data up to provide custom reporting for your users (customers) without giving them access to analytics is key to delivering metrics and performance