I get the same kind of flak for taking fortify every game. It's my favorite summoner spell. People always nag me about it. But every single game I get 2 or 3 kills on early tower divers, or save teammates in other lanes when they're in trouble.
My Mundo build
1. Warmog's
2. Spirit Visage
3. Merc Treads (if they have a lot of cc) or attack speed boots
Finish with any combination of Randuin's Omen, Thornmail, Force of Nature, Phantom Dancer.
Against a balanced team, I will buy Force of Nature and Randuin's Omen.
If they have a really hard carry that's doing lots of damage and lifesteal, I'll buy a Thornmail. If they have tons of AD, I'll throw a Sunfire on top of this.
If they have lots and lots of AP burst, I will buy two Warmogs, Merc Treads, and Force of Nature
Phantom Dancer is for when you have enough survivability to wade through the enemy like a kiddy pool.
Keep in mind that I haven't played Mundo since the Randuin's nerf, so I don't know how effective that item will be anymore. You might want to reconsider that one.
Trust me, you'll do enough damage to matter, and mid-late game you will never die. Early game you will rely mainly on cleaver spam for damage and survivability, and you can even use it for safe farming if you're unable to dominate your lane. In team fights, just target the enemy team's squishies. They'll run or they'll die. Either way, you'll break up the enemy formation and keep your teammates safe if they're smart enough to know how to take advantage of the chaos you'll cause. Make sure to keep tossing cleavers and activating masochism every time they're off cooldown, and keep burning agony on because it reduces cc on you. Maneuverability is really important for you, especially at low health. The enemy is going to be pissed at you and get greedy, and you can take advantage of that often. Cleaver chasers as they follow you, and you'll have time to regenerate until all of a sudden you turn around and kill them. The thing about Mundo is you kill your enemies by simply outlasting them. They will do more damage than you, but you will be regenerating all the time and if you can seperate yourself from the fight when you're low, you can kite people or simply stand to the side chucking cleavers until you're ready to fight again. This is why I recommend Phantom Dancer over Atma's. Great attack speed + high crit chance boost feels like it amounts to more damage than flat damage and a small amount of crit chance. Plus, the maneuverability is amazing. With PD you can wait until you are at like two bars of health in a team fight then activate your ultimate and you are ZOOM gone. Then you can literally be back at full fighting capacity in two seconds.
Mundo is a siege engine.
If you're facing ignites, the trick is to scare the enemy into using it early. It only hurts if they put it on you when you're activating your ultimate. Ideally, you want to save your ultimate for when you're low on health. Most players if you make some super-aggressive moves at the beginning of a fight will panic and use it on you too early.