Ask HN: Do you want a service which will alert you about an airfare drop?

This is a question mostly to travellers, but maybe everyone else will find it useful.

A part of our travel is a process of finding the lowest prices for tickets and the best connections. But sometimes the price itself is a good option to have a trip.

Do you think it will be useful to have a service, where you can enter your departure / destination city and subscribe to an airfare change?

Do you use any existing service?