It's certainly something that'll keep you playing an extremely long time, especially given the high difficulty level of much of the rest of the game. (gamingreinvented.com)
Thankfully, the game can be played in higher difficulty levels with all the upgrades, so at least that's a plus. (glitchcat.com)
My question is will there be different difficulty levels for different sets of players? (blog.us.playstation.com)