Ubiquitous computing environments advocate creating applications by plugging together various resources (mobile devices, displays, augmented everyday objects, and so on) and Web Services to support the user's everyday activities and needs. This approach is referred to as application composition, and such applications are called composite. Due to the dynamic nature of ubiquitous environments, application composition has to be supported at runtime, so that the applications are able to adapt to the situation within the environment and other contexts. Application composition is usually performed by autonomous context-aware mechanisms that provide limited or no control for users. Still, users need to be aware of their environments and be able to control and configure applications when they are composed and executed. Towards this goal we present in this article a context-aware application composition system based on the iCompose interface for composing and controlling applications at runtime. Users compose applications by simply touching the resources in the environment with their mobile devices, while the iCompose interface provides feedback and assists users by suggesting possible further actions. The interface relies on a rule-based reasoner and utilizes various context sources to support users dynamically, according to the situation in which they compose applications. We present a complete implementation of the system and report the results of a user evaluation study conducted with 21 participants. This study assesses the issues of control, usability, feasibility and user acceptance of the iCompose interface for context-aware application composition and the prototype as a whole.