I suspect how much people like this game depends a lot on what they are looking for.
I found the plot tedious and forgettable. The main character is an unlikable generic smartass, the supporting characters are clownish cliches, and the writing suffers from extreme cleveritis. The only interesting plot element occurs in the first half hour of the game. And like all GTA games it suffers from an extreme disconnect between the way your character is portrayed and the way he acts in-game as a deranged sociopath who makes Charles Manson look like June Cleaver.
The drug dealing aspect left me cold - wait for hot tips on cocaine then buy low / sell high. And unless you're a completionist there's not much to spend your money on anyway.
That said the actual gameplay is fun, the missions are creative and never fall into a familiar stock-mission rut, and what Rockstar has accomplished with the game technically is pretty remarkable.