Wow, this game is awesome. Technically it has some problems. I heard the game is not very well optimized, but i can play at 30-40 fps at full details on my pc, fair enough. Many textures, especially on the characters, are ugly. It's lacking a lot of effects (no normal mapping, few pixel shaders). On the other hand, the amount of details is fantastic, and the world is completely seamless. The gameplay is very open ended. Haven't had so much fun since a few years.
The scenario hem hem.. i can definately see why the game is forbidden under 18. The beginning of the game, especially with the gang wars, is.. special. Swearing, cursing, insults, racism.. it's not something i particularly enjoy. But the scenario is getting better now (i just arrived to San Fiero) so all's good.
I didn't find the game very hard so far.