Disk Alarm only promises to do one thing, and the good news is that it does what it says it will do. I particularly like that I can choose which volumes to include or not include. In my case, I want to keep an eye on an external drive, not my startup drive.
The reason I only gave Disk Alarm four stars instead of five is that the menubar icon is ugly (it's just a minature version of the app icon), and there's no option to customize the warning sound (which isn't exactly a sound I would equate with a warning). How about a choice of a simple coloured dot or something for the menubar icon, and a monochrome option for when there's more than the minimum disk space (have a look at SMARTReporter for an example of this)?