When Saints Row came out, I was still young and impressionable, and when I saw videos of the game, I thought it would be too wacky to be palatable, so I skipped it.
When Saints Row 2 came out, I gave it a few months before I picked it up for my Xbox... and got hooked to it, logging some 50+ hours into my save file.
As soon as I heard that Saints Row the Third was coming out, I was ecstatic and preordered that shit immediately. My money payed off.
Saints Row the Third is a bit of a departure from the first two games in that, instead of having 3 plotlines directed by which gang you're pursuing, there's now one overarching plot that ties all 3 together, along with the new anti-gang military. Personally, I like this sort of style, as the "choose what gang to beat down" method of SR2 was a bit wonky, especially when an objective for the same plotline was suddenly on the other side of the city.
The extensive character customization was also fun; I feel like THQ and Volition fixed that up nicely, especially with the new options for clothes. The ridiculous factor has definitely been upped, and for the better.
I won't go into too much detail about the plot, but the fact that you get to fight off zombies, ninjas, wrestlers, and programs in a Tron-style pastiche helps with the flavor of this game. And the fact that you get to do it with a giant dildo, a laser rifle, a brain-controlling-octopus launcher, or even just a plain old airstrike doesn't hurt.
All in all, I'd say that SRIII is a marked improvement on an already stellar game. I don't know where Volition can go with SRIV, but I can only hope it involves space.