API with reliable address callbacks? I want my app to be able to create addresses and be notified when an address receives a deposit. What I've tried so far doesn't work reliably enough.

I've tried the Coinbase API (address callbacks didn't seem to work at all) and Blockchain.info (sometimes it works fine, but usually it takes several minutes to several hours for the callback to fire).

Are there other APIs that offer address creation with callbacks that don't involve running my own Bitcoin client, or is running your own client pretty much a necessity for reliable notification (within seconds)? Is anyone actually using Coinbase or Blockchain successfully? My service would need rather immediate notification of transfer (number of confirmations isn't important).
The Mt. Gox API allows you to do these things. It allows you to create new deposit adresses and withdraw/deposit money into existing ones. 

While the API dosent actually have something to tell you when your specific order has been executed, you can get a list of your open orders and compare those against a local listing of placed orders to determine which have been executed. However their websockets API will tell you when any trade has been made on the network which you could then check yours against as well. 

Another good API solution would be bitstamps though theirs donsent have a websocket component.