I've already spent more than 300 hours in this game and i'm still having a blast, once you've completed inferno the only thing you can do is farming better gear or gold(which might sound boring - but it really isn't). If you mix up your skill build and change the places you farm at the game will be more fun. I'm switching between two different characters and farming act 1/2/3 and often trying out different builds and so on. The game is only repetitive if you make it to be. The 1.0.3 patch made a lot of odd builds viable and it's viable to farm in all acts now, before 1.0.3 I also felt that the game was repetitive but now it's alot better. It's the same thing on other games, a few examples; lol/hon/dota gets boring if you're only playing the same champion. tibia gets boring if you're only lvling at the same place and never pvping. And there's always hardcore mode too which is fun to play after you've reached inferno in softcore.
Try out the starter edition first, i'm sure someone on otland can give you a guest key and then decide to buy it if you're afraid to "waste" your money. Hell, you might even profit because of RMAH, good items are still worth more than what the game itself costs.