I'm playing this game at the moment, it's very good fun but I think I marginally preferred Saint's Row 2. SR3 has spread out each of the side missions, so instead of just finishing one level and continuing with the next, you have to drive to another location in the city to continue. In one sense it prevents repetitiveness but in another it just breaks up the gameplay mid-flow.
SR3 also seems to be missing some of the better side-missions from SR3, like the bodyguard missions where you had protect a celebrity from their rabid fans by throwing the fans in front of trains, into lumberjacks with chainsaws, into the blades of a helicopter, etc. Also missing so far are the sewage spraying truck and the police brutality side-missions.
Still, lots of the good stuff is still there. Cars can be very heavily customised, the custom character creator is fantastic, there's loads of silly costumes to wear and so far I've still not run out of things to do. Driving a tiger around in a car is a favourite new addition to the game.
My only really serious gripe is that it's crashed on me quite a few times now, to the point where I'm having to remember to manually save after every missions. I'm not sure if that's because my 360 is on its last legs or because the game was rushed onto the shelves. Halo Anniversary hasn't crashed on me so I'm more inclined to blame the game at the moment. Hopefully there will be a patch at some point.