Apigee isn't only for API providers - if you use APIs in your mashup, mobile, or social app you can monitor those APIs as well.
For example, if you're an iPhone developer you know nobody will use a slow iPhone app.
mLocal makes heavy use of RESTful APIs for content and especially to communicate to a back-end content app hosted on AWS. Shorepoint uses Apigee for monitoring and debugging of these API calls between the iPhone client and AWS (in both QA and production), and especially to monitor response times and proactively find out if any API call is slowing down iPhone app performance.
Thanks to Rajan of mLocal for all the feedback on Apigee and check out mLocal here!