I've been using this for over a year and Pester has become indispensable. What I needed was a reminder app to alert me to bill paying, picking up my son at school, appointments, projects due, kitchen food, ebay bids, ... whatever I need to remember. So while I guess this could be considered an alarm clock, it is more about creating both one-time and repeating Reminders.
I tend to set the alerts to overkill by bouncing the Dock icon, displaying a message, playing a tune, and speaking the alert message. Because I'm not a clock watcher and I get distracted by my work projects, this gets my attention. Plus I can click a snooze feature to re-remind me of the alert if I'm not ready to deal with it yet.
Lastly, I can save all my reminder messages in a list to choose again later, as well as delete my "one-time" reminders. Pester's interface is flat and ordinary—which would be nice to see improved—but it sure gets the job done.