I'm doing system testing for 1.5 now, and just used my first real event driven action just a bit ago, in my own personal system. Pretty neat. I set it up to watch for a power on of the Lexicon and power up the home theater in response. It's too bad that I don't have more stuff that I could put the event system to use on, but for me it's fairly limited because of my particular setup. I guess that's why it never seemed to be a paritcularly important feature to me in the early days. I can definitely see though how powerful it could be if you had the setup to make use of it.
I'll get more use out of it post-1.5 as we get support in place for some DVD changers, and I implement an on-disk music repository in my own system. Then I'll be able to get events like lowering the lights at the start a movie, or sequencing some trailers before the movie, raising the lights as the last chapter of the title starts (should be the credits), and so forth. But I don't have any motion detectors or security sensors and I don't have HVAC under control because I really don't have any HVAC (just a baseboard heater, where barely gets used in this climate.)
But anyway, it's there and it work well, and you can do very specific filtering. The response is pretty instantaneous. It is a network broadcast based mechanism so it is not 100% guaranteed that the event will be seen by every listener on the network, though it's very unlikely that they won't as long as the network is functioning properly and it's not being pushed to near its limits.