City Bunkers

So, I just watched a guy build a city Bunker to level 5 in Berlin and put his 25-stack into it. I assume he thought the Level 5 bunker would "protect" him. Then I watched another player roll up with his 40-stack filled with Arty and Rockets and spend the next few hours pounding Berlin to dust. Got me thinking. Should you even bother building a city bunker? Is there a better strategy for building and using bunkers? Are the Bunkers Under powered?

Thoughts on city Bunkers?