One of the most popular item in Season 3 and for a good reason. Since the change of armor and magic penetration mechanic, % number will take affect first then flat number, which increase overall damage even if you stack armor or magic resist.
Also the ‘tweak’ on warmog’s health regen, 1% of maximum health, if you stack health on champion (Olaf or Shen for example) it will give a huge amount of health regen which make you almost unmoveable in lane. It is 10 health regen per 5 sec from the warmog’s itself plus 1% of champion health (based and other item combined).
For me, I prefer Warmog’s Armor compare to Guardian Angel. Because the reduce amount of armor and magic resist since the ‘tweak’ and in team fight if your team is losing when you revived, you will be surrounded by enemy. So I believe, having extra health is better to last a little bit longer and do more damage to the enemy, instead dying and make a 4v5 situation in team fight until I revive again.