Having spent 40 hours beating the game, I say it's 75% pure awesomeness, 25% slight disappointment. The disappointment comes down to the story and activities.
On the story front, it's a case of stuff not quite having the impact they should from the death of a major character to the ultimate showdown. Everything works well enough as a collection of wild-ass missions, but I can't ever say I was particularly drawn in by any of the story threads and the new characters, while cool, just don't have the same impact as previous introductions, such as Shandi. On the subject of Shandi, the actress change was a bit of a mistake. Eliza Duschku was pretty much pitch-perfect as the happy-go-lucky stoner party girl who wasn't really hardcore enough to be hanging with the Saints... and while I'm cool with the more upscale version of the character, none of the previous character beats seem to be there, as the new actress only seems to be playing the badass on a vendetta role, perhaps not realizing that she's basically there to get kidnapped.
As for Activities, in previous games the Activities have been either in tune with the main missions or more over the top. Most of the activities simply carried over from the second game and are a massive come-down from the main missions (doubly so for the Activies that are Main Missions). Professor Genki, Tank Mayhem, Tiger Escort, and Heli Assault are really the only ones that fit in with the campaign. And while I'm not really a big fan of the first two games having six instances of an activity at every location (this makes it a huge grind to completely finish the game), the one per location (and all of them being pretty easy) meant I actually finished the game to 100% in 40 hours instead of having lots of content. Having had a blast with Whored Mode, I wish they had brought that into the main game, because beating gut-toting hookers to death with a giant dildo would have made a great activity. Likewise a rickshaw race or fighting giant mascots.
But I forgive the game for these minor flaws for the ability to jump into a freakin' car. I can't tell you how irritating it is to be on a serious time limit, baddies gunning at you as you race for the car, then you slowly open the car door and step in... totally ruins the mood of the scene.