I've beat it twice now. Second time was just by accident...me and a friend just kept playing missions for a couple days, due to boredom, and beat it.
It can get annoying at times but I always thought it was decently easy. But I love the game either way, just so much to do and it looks so good too.
GTA Online is probably the best part of the game, in my opinion. Just started playing again recently because I stopped due to all the people hacking it and giving every one billions of dollars. But I'm glad they fixed that.
And c'mon! GTA 4 was fun!