Simple, dependable, nice-looking. I like the built-in alarm sounds, too, which are pleasantly subtle. Thanks to the developer for their fine work.
I thought I'd clarify a couple of things that may not be obvious:
• An "Alarm" is like an alarm clock: You set it to go off at a particular time of day (using the 0-24 hour format), on one or more days of the week. It takes effect as soon as you create it. You can click the slider control next to it to turn it off and on again.
• A "Timer" is a countdown timer: You set it to run for a specific length of time (minimum 1 minute) till it goes off. However, it does NOT start automatically; you must click its "play" (triangle) icon to start it.
Maybe the reviewers who thought the extension had "stopped working" didn't realize they had to re-start their alarms/timers to run them again. (That said, Alarms _do_ restart automatically if you select "Repeat Weekly" when you create them.)
The extension's only shortcoming, IMHO, is that the alarm window always appears in the upper left corner, even if you drag it elsewhere before closing it; it'd be nice (and more standard GUI practice) if the extension "remembered" its position. Also, the "vibrating alarm clock" animation seems like overkill---it really necessary, in addition to the window appearing and the sound playing? Maybe it could be an option, for users who like that sort of thing?