Some data to consider:
Advantages: Essential if its niche of business concentrates in Catalonia. Very either accepted by the community of Catalan speech and it very complements or with other domains (.es, .com, etc) in which to center the rest of languages.
Disadvantages: Very young, it is an extension not very well-known outside the Catalan community.
Something of history:
The domain .cat was born in September of 2005 to serve to the needs of the linguistic and cultural community as Catalan speech.