Rockstar seems to have taken this approach with their games recently. San Andreas, GTA IV & Red Dead all have slow starts. The issue seems to be whether you dig the protagonist and his story or not. I didn't really care for San Andreas' protagonist or story until much later in the game while both GTA IV & RDR struck me as immediately being interesting despite the slow build up. Heck, in GTA IV you don't get a gun until a couple of missions in.
There is nothing wrong with the silliness of Saint's Row, but I prefer Rockstar's Godfather approach opposed to Voalition's Don't be a Menace to South Central While Drinking Your Juice in the Hood approach. Also don't know why some are saying GTA IV wasn't fun to go nuts in. I'd say it was the absolute best GTA game in the series for going postal in. Just watch any one of hundreds of youtube videos on the crazyness of GTA IV.