I think it functions as an adjective, though Eliot is right in pointing out that 'value' is usually a noun. Does the phrase describe the noun? Then it's adjectival (ie. it's not "an adjective" - a single descriptive word - but the phrase functions as one). So the guesthouse becomes a "good value" guesthouse. However, "good guesthouse" is very vague, and "value guesthouse" makes little sense. the adjectival phrase can't be cut down.

As a slang it could be considered collectively a phrasal adjective referring in your example to a place where hanging out is fun , but as far as correct grammar is concerned, 'good' is an adjective and 'value' is a noun.

No it is not. It is a noun, joined with another noun and thus becoming a modifier, but not an adjective.