にしては is used when you evaluate something based on a certain standard or expectation, and then point out that reality differs from what you'd expect. It often carries a nuance of surprise or criticism. For example, if someone is a beginner but speaks well, you say 'beginnerにしては上手だ' meaning 'good for a beginner'. The standard can be an age, a status, an amount of experience, etc. The pattern highlights the gap between the expectation and the actual state.

Note that にしては is different from わりに, which also expresses a similar idea but is less strong in its contrast. にしては often implies a specific judgment, while わりに is more neutral. Also, にしては is used when the basis is clear from context, and it's not used for hypothetical situations. It's fairly common in both speech and writing, but in casual conversation, you might hear it less often than in formal contexts.