StatHat makes it trivial to check if a stat goes above or below a value, if it changes more than a percentage from one interval to the next, or if no data is received for a period of time.
In addition to alerts, you can set up email reports to send you a daily or weekly summary of a group of stats. You can include other recipients to get the reports.
You don't need to worry about setting up manual alerts. StatHat constantly analyzes your stats and learns what is normal for each stat. If it detects anything out of the ordinary, it will send you an email alert.