First up, for £22 this is definately worth a flutter. However comparisons have to be drawn with GTA4, which although ultimately personal preference, do highlight some areas where Saints Row 2 is distinctly lacking.
The main difference is the technical limitations of SR2. Visually its showing its age, with barely any improvement over the original game. The city is chunkily rendered, but could benefit from better texturing and lighting, and large swathes are left fairly blank with little in the way of side details to notice/smash into. The framerate occasionally descends into single figures and draw distances can be a little short. SR2 also suffers from the incredible disappearing cars glitch that GTA used to struggle with - ie get ready steal an approaching car, turn around to shoot someone for example, turn back and the road is empty. Driving the cars is entirely personal preference, but I much prefer the weighty realism of GTA4. Cars in SR2 are utterly lightweight, with neither heft nor sense of momentum which leaves them feeling slightly bouncy. Handling feels a little odd, powerslides especially look and feel incorrect. Its still amusing bouncing around the world running over hundreds of pedestrians, but more gravitas here would have been good.
The initial character creation is a work of complete genius - if you can imagine it, you can create it, and its utterly hilarious. If you've ever dreamed of being an obese transvestite dressed in boob tube, lime green hotpants, flip flops and complete with crazy beard, dreds and manic expression, then yearn no more. You can even customize the walk to something far more ridiculous - the pimp walk for example. Its disgracefully brilliant and adds an extra dimensions that is always missing if you don't quite identify with a set main character.
Combat is identical to the original game - good enough to get by with enemies dying spectacularly, and all manner of items in the world to lay the smack down with - particularly fun is taking human shields to avoid damage before flinging them into a building/off a roof/ under a car.
The whole world is crammed with a mad exuberance about everything you do, its filled to the top with fun and immediacy - diversions are everywhere and as you progress, the ability to gather homies and go on hell raising carnage missions in enemy territory is welcome.
Ultimately its a lot of fun, let down by a general lack of polish and some glitches and bugs. Theres a lot here to entertain and amuse, but its all fairly lightweight. if you're looking for the antithesis of GTA4's heavy, brooding presence then the vibrant, mad, utterly un-PC and gleefully OTT world of Saints Row 2 fits the bill very well.