it seems that the instawallet.com has been being mentioned on this forum a lot.

Could anyone tell me that is it trustworthy? why? Has any cheating been report on this forum?

I personally willing to be charged by some honesty bitcoin online wallet service provider, if the fee rate is about 0.05% per year or something like that, and they can charge per transaction about 0.01% per transaction. If 10% of the bitcoin were deposit on the service provider, that will be about 100m*10%*0.05%=5000 USD revenue per year for depositing, and tens of thousands USD per year for transaction. And the revenue will go up and as the price of bitcoin went up.

But guarantee for the safety and honesty is extremely important.