these investment websites are scams .. its like gambling u need to know the right time to withdraw.
I think this hugely depends on what kind of site you are investing your bitcoin in, and you should only invest in sites with trusted owners. Sites like Justdice and Moneypot (under the original owner before sale) have demonstrated that owners can resist temptation, run the site fairly and return bitcoin back to the investors when requested.
tell me this, do u know these people personally. which site or rather owner to trust which to not... how to decide. One can demonstrate anything over internet. its like magical trick.