Writing down your key is not a good idea as even one "typo" would cause your entire "key" (it wouldn't actually be a key anymore) worthless
+1
Absolutely. I used brainwallet and wrote down the private key. I knew the risk of typos, so immediately aftewards I used the option to type a private key in to get an address out. Well, mistakes did happen, like is this an "s" or "S" or "5". It took quite some trial and error to get it right.