Not bad at all...
The only games of the genre I remember myself finishing were Resistance 1 & 2 and Killzone 2.
So, after several days of GT5 (and been fed up with it) I rented it and gave it a shot, half-expecting that I would play for 1-2 hours and then return it.
Surprise! I loved it during the first twenty minutes. Okay, it did not "enchant" me with its graphics but after all it is three years old already. But the fact that you can destroy almost everything you lay your eye on is just extraordinary.
The realism is quite balanced. You will not die with the slightest hit or because you forgot to take cover, but then again if you lunge head first towards a well covered enemy, even moderately armed, don't expect to live more than 2 or 3 seconds.
Your enemies will take cover and cooperate keeping you busy while trying to approach flank and kill you.
Slight technical problem here, with enemies vanishing and reappearing especially in confined places.
The vehicles are simple to use, but they feel real unlike some other games where they could have been Star Trek's transporters. The only thing that I did not like was the fact that when my character drove your squad members would man the available weapon and fired it (usually missing) but when I manned the weapon nobody would drive the vehicle.
Your team mates are usually following you but you will not really feel their existence with the exception of some comments when you manage a good shot or when you forget to take cover and get hit.
The overall feel is immersive and I found myself cringing and ducking or raising my head from behind cover in order to get the enemy without exposing myself.
Can't wait to finish it.