would you trust an online wallet with considerable amount of bitcoins?
No, although I think blockchain.info is reasonably secure.
To store a "considerable" (definition subjective) amount of bitcoin, a cold wallet as described in this thread is by far the best solution. Your hot wallet "probably" won't get hacked, just like the pile of cash or gold you store in your protected-only-by-padlock garage or shed "probably" won't be stolen.
Spot on. You're only secure if breaking your lock takes more effort that the perceived value behind it. If nobody know you've got gold in that shed, it's (mostly) as secure as if it was in a bank vault.
If everyone knows... It's another story. =P