There are many kinds of users, the end-user, the company, another company using your services, even an organization and departments. Each has different requirements, different effect to systems and "user" satisfaction, etc. And then there is the most important "user" - the business (and profit) goals of you company!

So - not so simple! APDEX is one way to formalize the performance measurement but.. Asking an "application" performance is a little too wide question today - previously an application was just maybe "one" program running or a bunch of transactions. Today an application touches (most of the time) several parts of the whole infrastructure and that's where the problem starts.

Too many companies still live without (business) capacity planning or really don't understand what it means. They just run performance numbers, try to speed up one part of the whole and often end up even worst problems they had previously.

Yes, most important is the "user" but, as in computers, priority alone can not be used for performance management because nobody has unlimited resources. Yes, like largest companies, simulations and tests can be done, but how many know how? Technically the most important, I think, is not to corner yourself without plan B. If usage performance would be easily predicted we could do the same with stock and start trading futures.