Decent gameplay marred by complete unoriginality and a few annoying elements.
That said, Saints Row does make a few changes from GTA, and some are definitely improvements. The targeting system, though not perfect, is still much better than that of GTA III, Vice City, and San Andreas. The lines on the maps showing you how to get where you want to get to, is also incredibly helpful. The city is sufficiently large, though it lacks the distinct character of Vice City and San Andreas.
The missions vary in difficulty, but only occasionally are they hard enough to require more than 2 or 3 attempts to complete. That said, the missions tend to be longer the GTAs missions, and a lack of any checkpoints (and frequently there are many perfect places they could be put) is frustrating when you're forced to redo something you've already done, easy or not. The controls in the many "shoot-at-people/vehicles-while-someone-else-drives" missions are also clumsy and annoying making those missions probably the worst in the game.
Another nice touch is the ability to collect "homies" and call them up to help you fight. The ability to revive them should they die is handy. Another nice touch is the regenerating health which helps prevent frustrating deaths since before engaging a new group of badguys you can always stand back and wait for you health to go back up.
The game suffers from some annoying loading glitches which, on multiple occasions, locked up my Xbox to the point where the only solution was to turn the system off. Despite the release of a patch, these didn't go away, though for whatever reason they seemed to happen less and less as the game went on. Perhaps just a coincidence. Regardless, it was incredibly annoying when such things happened during a mission, especially when I was already well into it, which occurred on several occasions. Some other glitches forced me to restart missions when the sound disappeared (another loading issue) which prevented mission objectives from showing up on the map, making them impossible to do. Another time, a character got stuck in a wall, but I was required to rescue her which then became impossible. It really feels like the game was rushed out and not polished as well as it should have been.
The story is also not very compelling. It mostly consists of taking over territories of rival gangs until you've taken them all. Each gang has its own subplot, but they're not terribly interesting. The story does start to get a bit more interesting before it ends, but is abruptly cut short by the ending.
The music on the radio is also terrible, so much so that I had to turn the music volume right down. There wasn't enough variation in the types of songs, and what songs were there just grated. I wasn't a fan of San Andreas's soundtrack, either, but it had a few stations and songs that were decent enough that it didn't get annoying. Perhaps its too much to ask that every GTA-style game have a soundtrack as good as Vice City.
Saints Row isn't bad, and being a Platinum Hit, can be picked up for a decent price these days. I'd recommend it if you like GTA, so long as you can deal with the issues I've discussed.