Most of the information everyone is giving you is incorrect. Bigger banks no longer "hold the check against your balance." The computer determines your usual deposits. So, if you deposit within your usual deposit range, there'd be no hold and the bank would assume the check was good since all of your other usual deposits have been good.

You assume full responsibility when you accept a check and deposit it into your account. This was diclosed when you opened your bank account in written form (all the paperwork given to you upon opening).

With Check21, checks will usually clear within 1-2 days at most.

Although it does suck, if you don't trust the check 100%, take the check to their bank, cash it, and then deposit it. That's the only 'safe' way of securely getting the funds unless you have a Cashier's Check or Money Order (certified funds).