It depends on a lot of factors for me whether or not I beat a game 100%.
I just picked up a PS2 and have been playing God of War, I'm on the final boss battle for God Mode. Definitely a great game, but God Mode in the game feels cheesy: the enemies didn't become more complex, just a lot longer to take down and they do more damage. Sometime around the 15th time you bounce the same minotaur it becomes boring and tiresome.
I may not beat Ares this time around, but I already have on easy, normal, and hard. God Mode was disappointing.
If you haven't played it, don't let my assessment of God Mode discourage you. That game alone was worth the price of the PS2. I swear GoW could almost turn me into a Sony fanboy :O
By contrast, Super Swing Golf was definitely worth getting 100% done as you have all the characters and items and a ton of cash to gear them up. It's great for playing with friends when you have like 8 characters and 12 courses (I think, I'd have to check again).
It really depends, but my main focus is local multiplayer. GoW was something of an odd duck since my wife really loved sitting down and watching just for the story.
3rd Party Wall of Shame
http://www.nintendoworldreport.com/forums/index.php?topic=30478.msg581036#msg581036







