Although I never used them that analogy is not quite accurate. It's not like handing money to a random stranger around the corner. More like trusting a known dealer. Sure, if you trust someone there is the possibility that that trust s betrayed. That is the definition of trust.

Is instawallet a good entity to trust money? I would say, they appear to be a serious website, however the concept is, by design weak. I deferentially  prefer not to rely on someone elses trust. Also, what if the server gets hacked?

if you need to recieve a payment quickly and since you are copy-pasting addresses and keys around, why not generating an address in, say bitaddress.org?