I wouldn't call this a GTA killer by any means. I'd rather just wait for the next GTA on the 360!
The two games are similar, but whenever I described Saints Row to someone I'd always say, "It's like GTA and True Crime combined."
The sandbox elements are there, however somewhat fleshed out. For example, in the demo you can rob different stores. You have to point your gun at them and wait for them to put their hands up. Afterwards, you have to walk them to the safe and get cash. However, if you keep your crosshairs off them for too long they'll run and turn on the alarm!
I was kind of impressed with this. However, other than this and a few other nuances the gameplay isn't breakthrough.
The whole gang on gang violence premise isn't worthwhile, either. =\
Jam it back in, in the dark.