So I've done just about as much as I can in Saints Row 2; I'm cashing out at around 70% completion, and while I haven't finished all the activities (and the Achievements that accompany them), I've bought pretty much every store in the game so I'm never going to be short on cash.
The endgame in SR2 is basically just one big crazy fever dream; there's definitely a lot more to do than in GTA4, but it must be said that Stillwater isn't nearly as interesting to explore as Liberty City is. And because SR2 is so much less punishing than GTA4 in terms of death and/or failure, it eventually feels a bit too arcade-y. Which is fine, I guess; it is its own thing, now. The first Saints Row felt like a direct challenge to the craziness that was San Andreas, but GTA4's change in focus was so unexpected that Saints Row 2 really feels truly different. GTA4's seismic shift in tone was so radical that I'm not sure that the GTA franchise can ever go back to the over-the-top craziness of its earlier games; and as such, Saints Row now has the green light to basically go fucking crazy - which, to its credit, has always been its focus anyway.
And so I am curious as to how the franchise will evolve. SR2's story (and city) is a direct continuation of SR1 (however improbable that may be), and your character's journey by the end of SR2 is pretty much as far as that character can go. If we are so fortunate as to get a Saints Row 3, I really only have two requests: we need a new city to play with, and the game will need a bit more technical polish. (Also: the Achievements could maybe be a bit less grueling. I sunk 20+ hours into the game and I've got less than 300 points to show for it.)