I suggest you download demo for a quick taste .
On what platform do you intend to play ? I'm on XBOX , so that's my experience.
Free roam is great if you enjoy exploring , getting milestones for cars , unlocking upgrades ,jumping, drifting , crashing billboards with your friends faces all over , avoiding traffic , taking speed traps ... and of course Cop Chases :D
if you got few friends that also play this game you will soon find yourself chasing their records on cop chases, races , speedtraps ,bilboards ...
It is fairly big world and there's quite a lot to explore > from highway to offroad dirt tracks , drain tunnels , different city parts , parks , buildings ,construction sites ... name it ,it's there .
Cop chases are engaged if you do high speed drive by, donuts/ drifting or if you crash them . Then it's up to you will you run or will you continue crashing ,or whatever you wish ( my advice is > do some car upgrades first ).
There is 6 levels of heat and higher the level is , more chalenging/numerous cops become .
Cop cars come in few diffrent types > normal, heavy, sport corvette with spikestripes , SUV & SWAT truck which must be "the hammer" you asked for
I think longest cop chase I had is around 25 minutes and numerous crashed cop cars and roadblocks. Cop chases can last as long as you wish > keep a small distance , don't get caught and it will go into infinite :D You acumulate points for things u do (milestones,traffic takedowns,cop takedowns, evaded roadblocks ...) while being pursued . Trick is that if they get you ,then you lose all acumulated poins and you're teleported to jack point where you found your car.
however , there is no cops on-line , so if you get frustrated by copschases, you can always go online for some more FREERoam :D