Experiments with a semi-autonomous wheelchair controlled by means of a Brain-Computer Interface (BCI) are presented. The navigation system, having at its core a collaborative controller, performs smooth and safe manoeuvres following sparse steering commands provided by the user. The user intents are decoded from electroencephalographic signals evoked by a visual P300-based paradigm. Experiments have been performed by several able-bodied users and motor disabled participants, showing the effectiveness of the approach.