The main benefits of a complete quality catback system are that they are bolt on, better quality stainless than stock, mandrel bent piping, and they also include better quality (and usually larger) exhaust tips. From a performance standpoint, you wont gain more than 1-2hp over a standard muffler swap.
The tips alone are worth it to me. Adding 5" tips usually runs upwards of $250-$300 for the tips alone. By the time you factor in also adding a new muffler and having it all welded in, you are back in the same price range as the bolt on catback system.