I've played many, many, many games with Mundo top and let me tell you first hand: it is FUN! Mundo truly is a fun champion to mess around with. There's some points in the game where I feel as if I'm trolling the other team when I use up all their CC and damage abilities in a team fight, walk away with a good amount of health left, and since I'm still on my ult, I run back in with another 3/4 health left.

While playing Mundo early game, make sure you are careful not to become too overconfident. Mundo CAN die, but if you play it smart and you don't die...Mundo becomes basically impossible to kill around the time he gets Spirit Visage (or a little before sometimes) using this build.

When you're top, focus on trying to hit your opponent with as many cleavers as possible, harass the heck out of them. if you are playing it safe with lower health, use your cleaver to get long distance minion kills (especially on the siege minions for extra money).

Movement speed for my quintessence? But Fobbos, why?
Here's why: 4.5% percent movement speed may not seem like much, but when an enemy AD carry is about to get away and you give chase and just barely get in range to hit with that cleaver and slow them down to finish them off (or allow your team to finish them off)... that extra speed can be a real game changer. Plus, offensive uses aside, having the movement speed helps a little considering you don't get your boots for a while in the build.

Why you ask? Why do I have 29 points in Defense? Listen to this: Mundo can become nearly invincible. the few extra bits of AD, AP, or Magic pen he could get with points in Offensive are nothing compared to the bonus of staying alive in the game, especially in consideration of the rest of your team. These masteries help keep you alive in congruence with this item build.

Basically focus on your cleaver first. This allows you plenty of poke damage as well as later becoming great for slowing enemies in order to catch up to them for another hit with masochism, burning agony, and then another cleaver. Level up your skills with this in mind:
Sadism > Cleaver > Masochism > Burning Agony

I would have to say there are very few things in this game that can outrun a Mundo with alacrity boots, Sadism activated for +35% movement speed, and ghost activated for another +35% movement speed (normally 27% but the one mastery in Offensive gives you a bit more speed). Utilization of this combo along with cleaver will not allow many, if any, enemies to outrun your Mundo. I've often dived enemies inside of their own base (not fountain obviously) while they were still trying to run away.