I’ve spent much of the last couple of hours playing around with Growl. (The balance of the hours was spent clearing up divisibility confusion and evacuating the building.) It’s a nice idea in much the same way that RSS is a nice idea, in that notifications are exactly the sort of thing that should be standardized (but with the option to tweak). It’s also the sort of trick that the archietecture of OS X/Cocoa makes very easy to do, which means low overheads in processor time; this is a significant concern on my office machine, which has never really had as much memory as it needs to support my megawindowed workstyle.
The only issue I’ve had with it is the GrowlMail plug-in to interface with Apple Mail; installing this causes Mail to crash upon opening, which is annoying. Browsing the fora suggests that this issue comes up sporadically, but there doesn’t seem to be a single “here’s the problem” kind of solution for it.