Masses of Fun
Each gang boss is (If you do them in the correct order) harder than the last, which is where a key element to an otherwise boring game comes into play. By using different methods of eliminating gang members and bosses, and doing other things on the side, you can make your character better at certain tasks. Agility orbs (located on top of buildings etc) allow the charachter to run faster and jump higher. Driving through htunt rings, or running over gang members makes the player better at handling cars, as well as eventually being able to enhance agency vehicles. Perhaps the most comical of the 5 skills, a high "Strength" skill meter allows the player to pick up cars, people, and a whole bunch of other stuff, and throw them far far away. Explosives and Firearms skills speak for themselves.
You might think that this is all there is to it (Which, if you are like my friend JoeTonks, it is), but as an Achievement Whore, I am having loads of fun just levelling up my character, playing around in the Agency SUV (It can go up vertical walls when you achieve a high enough Driving rating), and generally just pissing about.
Hardcore gamers may be put off by how it isnt ridiculously hard, but from my point of view, that is a good thing.