promises vs regular async

vs.js

eth.getKey(function(key) {
  eth.getStorageAt("someaddr", "somelocation", function(address) {
    eth.compile("thiscode", function(code) {
      eth.transact({to: address, from: key, gas: 10000, gasPrice: eth.gasPrice, data:[code]}, function(tx){ 
        console.log(tx)
      })
    })
  })
})

// VS

eth.transact({
  to: eth.storageAt("a", "b"),
  from: eth.key,
  gas: 1000,
  gasPrice: eth.gasPrice,
  data: eth.compile("code"),
}).
then(function(tx) {
  console.log(tx);
});

Your second example with a callback.

eth.transact({
  to: eth.storageAt("a", "b"),
  from: eth.key,
  gas: 1000,
  gasPrice: eth.gasPrice,
  data: eth.compile("code"),
},
function(tx) {
  console.log(tx);
});

There are many better ways of demonstrating how promises are awesome, but in your example promises are actually not needed.