It has good all round armor to fend off light tanks and some weaker and low tier mediums, but that's that. I used to own one myself until I sold it for something bigger, but I really used to like the Churchill.
Your job with it is to crunch down smaller tanks, as the high firing rate of its small gun is perfect for the job. Do not try to take on larger tanks, like a KV or KV-2 especially if they are facing you with their front armor, because you won't penetrate all the time, while they stand a much larger chance at penetrating you from any side. It's a support tank, don't use it on the front line.
About the weak spots and as others have mentioned, the turret. Make it a habbit (with any tank) to always try to point your front armor towards the attacker/s and angle yourself. Give the churchill a chance, remember the credits it's going to make you.
Edited by blackfox666, 17 September 2012 - 02:07 PM.