As in all exhibits concerning literature: more reading than seeing.
However there is a wonderful little movie depicting Prague at Kafka's time.
A lot of information about his life, his relationship to his father, his abhorence of his job in that it kept him from writing, his relationships with women.

Kafka is not exactly known for writing light-hearted fiction, and this superb little museum is built to match that mood. Visitors walk through lengthy, dark corridors with exhibits of letters, photos, and other documents (it is clearly stated that they are facsimiles, so, no, they are not originals, and no attempt is made to convince visitors otherwise) that serve to contextualizing Kafka. In a city that seems to encourage entrepreneurs to open up shady, overpriced, and disappointing museums in any spare room they may have available in their apartments, this experience came as a most welcome surprise. We were, however, initially suspicious about the place due to the fact that we were “greeted” by a rather unpleasant employee at the reception desk; we prefer to assume that her vacant and disagreeable manner was an act to make our visit more Kafkaesque.

Extremely interesting museum - I was the only one in the group who had read Kafka but we all found it highly enjoyable. The original letters were particularly special, but all in all, this is a must-see if you're in Prague. The admission cost was €8 each.