This article presents a speech-based user interface to a wide range of entertainment, navigation and communication applications in mobile environments by means of natural human-machine dialogues. The underlying dialogue concept has been developed in the framework of the EU-project SENECA. The novelty relies in the fact that speech recognition inconfidence and word-level ambiguities are compensated by engaging flexible clarification dialogues with the user. The SENECA system demonstrator has been evaluated by means of user tests. Some recent evaluation results are discussed in the paper.