I have 300 hours in this game, I've done every mission. I've completed almost every mission on the hardest difficulty, the only ones remaining being the most recent paid-DLC missions that they added when I stopped playing this game. I've got to V-100 (Max level, max prestige) legitimately. This game is decent but it has far too many problems.
Firstly, playing with friends is a blast. Get together with 3 other people and rob a bank, diamond store, etc. Go in stealthily, silently take out guards in an empty corridor or just swagger in while putting a mask over your face and holding a shotgun. That stuff is great at first. If you're an experienced player and friends are new to the game -- Let them host the game and pick the missions. I see too many people catapulting their friends through the levels by doing max difficulty multi-day heists. A natural progression is much more enjoyable for everyone.