Un Chien Andalou and L'age D'or embrace all the principles of surrealism, and are vivid examples of the power of cinema to express the power, uniqueness and oddness of dreams. If you're looking for a normal, Hollywood style, plot driven movie, then these films aren't for you - there is very little, if any, plot to either of the films, and searching for meaning or attempting to empathise with the characters will only cause confusion. Surrealist films are intended to be viewed as explorations of the unconscious, and as sets of images that may or may not have relevance to each other. They were also intended as a reaction against Hollywood and its mass market appeal.
The first film, Un Chien Andalou, is only 16 minutes long, and is not for the squeamish. Scenes of eyes being slashed with razor blades, ants crawling from hands, severed limbs, and decaying animal corpses are all shown in close-up, so be prepared. The film is a confusing, viscious assault on the senses, and touches upon ideas of death, rape, sex, sin, violence, etc.
L'age D'or is longer at 63 minutes, and unlike Un Chien Andalou, which was a huge commercial success, it was banned almost instantly due to its blasphemous attitude to religion. It shows various tabboo events, including a man shooting his son when he irritates him, and is generally unpleasant, though intriguing. This film was disowned by Dali, who broke off his partnership with Bunuel afterwards, as he felt that Bunuel had destroyed his vision and created a film which he hated.
Personally I found both movies difficult and unenjoyable to watch, but they are fascinating as works which provoke questions of desire, repression, the connection between the conscious and the unconscious, the placating power of Hollywood, and the eternal question: What is art? I would only recommend them to people who have an interest in such questions, or who particularly enjoy Surrealist works.
A final note: this DVD edition is excellent if you're interested in the history of surrealism, or the relationship between Bunuel and Dali, and will be a huge help to anyone studying it for university or for personal education. Both films have interesting and comprehensive commentaries by Robert Short (who, despite his comical tone, clearly knows what he's talking about), and in addition, the first disc also includes a long and interesting introduction by Short which deals with all aspects of Surrealism, from its beginnings in France with Breton, to the ambivalent attitude of the surrealists and of Bunuel himself to the success of the films. It also examines the symbolism within the films. Admittedly, it could have been more engaging, as it merely shows Short's head and shoulders throughout, and it would have been more watchable had they included some different images. There is also a documentary on Bunuel.