When using Google app engine is there any benefit to use a CDN if i wanted my file resources as closer to users?
Answer
Well, it is all about your budget, geography and profiling. Google app engine is free, and if properly configured it serves your content very nicely to various locations around the world. Many people actually use the app engine as a CDN! (see here, here and here)
Here is a blog post comparing the performance of the app-engine, a commercial CDN and no CDN at all for hosting a static 1MB zipped file
Try this free service (from a CDN...) to see how your app engine application performs. Here is another test site I've used
So, depending on where your site is geographically located, where your users are located and what your needs are, you need to profile the performance of the app engine and see if it is good enough. Will you get better performance from a commercial CDN? Most likely. But do you need it?