After mastering all the jobs in a specific tier 3 times, you will master the tier, get a title and a reward item. Here are these reward items:
Master Street Thug - Pistol Bayonet: +4% damage done in fights
This is an easy one to get, and definitely worth it! Dealing more damage in fights is a great bonus.
Master Associate - Bugatti: -7% damage taken in fights
Also an easy one to get, and definitely worth it. Will win more fights with this item.
Master Soldier - Golden Skull: 30 second reduction on health regen timer
Pretty worthless. You have to master soldier to become a master in the next tier though.
Master Enforcer - Money Plate: 5% discount on property purchases
Not bad. Every little bit helps if you are on the quest for the "Nest Egg" achievement.
Master Hitman - Chainsaw Bayonet: 30 second reduction on stamina regen timer
Also pretty good. This item lets you attack other players more often.
Master Capo - State Senator: 5% discount on property repairs
Not all that great. If you are spending a lot of money on repairs, see the "How Not to Get Robbed" post.
Master Consigliere - Helicopter - 30 second reduction on energy regen timer
Lets you regen energy more often. Not bad I'd say!
Master Underboss - Private Island - +5% more experience points from jobs
This is the best reward item in my opinion. It makes leveling up a lot easier!
Master Boss - Golden Throne - 2x the energy regenerated per regen period
This is final reward item. I think it is bugged. Right now, you can only get 1 energy regenerated per regen period, so with this item you get 2 energy. However, at higher levels, if you watch your energy regen, it goes up by 2 (without the golden throne) but when you refresh the page, it goes back down to only 1. I think if the creators update the game at some point, this item might get better, and provide more than 2 energy per regen period.