blockchain info API is very easy to use, but I don't think it is secured enough - mtgox merchant API had a better security.
I mean: except for the fact that the money was going into mtgox block hole

anyway, the best API I can think of (which I don't think exists yet, at least not for free) should be based on public key cryptography validating the 'payment received' callbacks.
blockchain info (for all I know) uses a fixed authentication token, same one for everything - that's just a disaster waiting to happen, if you'd want to use it for a serious stuff.