I have installed one in my car as a warning for any failures of the VTEC, Lets you know certain things if pressure is not correct, or if the solenoid is bad, or if your low on oil.
The light will actually throw a "code" if it's not working properly. It's interesting, the code is not exactly readable nor does it change, it just blinks once per second to let you know something is wrong, and it's dim.
I used a HILED (blue) to light up once VTEC kicks in. It's a very simple mod, can be done for probably <$10 and 30 minuites. The last time VTEC didn't work a bolt was bouncing around in my valve train.... wasn't perty.