Warning to anybody considering using this, if you don't control the private keys, you don't own the bitcoins. This reminds me too much of inputs.io
Agree with RGBKey. If you don't own the private keys, you don't own your bitcoin! You are simply depositing money in a bank where funds can be seized, frozen, or stolen. In addition, they need to store most funds in cold storage for security thereby making transfers possibly take VERY long if their hot wallet gets drained by users.