Which factors influence user acceptance of Ambient Intelligence applications is an interesting question worth to be studied comprehensively. We describe a quantitative user study that investigates how experience, stress, and system behavior influence user acceptance of an ad-hoc assistance system. We find that stressed users perceive the assistance system as more useful than relaxed users. Furthermore, system behavior influences how useful people perceive the system, and experience influences how easy to use people find the system. Perceived usefulness also depends on how technophile a person is. Following our findings, we develop a scheme of user acceptance and performance.