In a survival situation ANY compass is going to help. Even the compasses you can find in a cracker jack box will tell you which way North is and give you a halfway decent bearing on where you need to go.
I strongly disagree. I've seen compasses not only not point to magnetic north, but almost directly south. Also, a "halfway decent" bearing might land you in hot water if you follow it long enoug.
Where I live it's really easy to tell if a compass is accurate. Down South here there is a moss and fungus that grows only on the North side of trees. I stand with my back to the fungus coated tree and hold my compass out. Accurate as a button compass can be.
The moss thing can tell general trends and be helpful, but not always accurate. I've seen plenty of ridglines that obstruct sunlight and change the formula a bit.
I've also wondered how much good any of these button compasses are without a decent map. <img src="/images/graemlins/confused.gif" alt="" />