Though this game feels very similar to GTA, this game have a lot of features that over-powers GTA.
One thing that GTA lacks is something that many people consider a must to have on a free roaming game. Customization. You get to fully customize what your character looks like from how much he weights to how high or low his eye brows go. You can even create the character to look similar to you.
Physics is another thing that this game overpowers GTA in. When a car hits you, a car is actually hitting you. You don't land flat on your back or on your stomach motionless. Your characters body will go flying with arms and legs flapping around. Hand-to-Hand is very awesome on here. You can hit with both hands and even kick your enemies in the crotch (or very close to it), which is very funny to watch.
Finally graphics. While this game is on a next-gen consol and GTA has yet to be on one, the graphics are great. I'm not going to go on bragging about graphics because GTA IV isn't out but I will be waiting to see what it looks like.
Saints Row is a very good game that should be played by anyone who enjoys GTA.