From my perspective, I don't think they're such a good idea. First of all, if my understanding is correct, oil pressure, and hence lubrication, is lowest on an idling engine, especially a cold one. It's an invitation to premature engine wear. What I have heard is that it's best not to let a cold engine idle for more than 30 seconds after start up, and then to drive off moderately until proper operating temperature is reached, which incidentally happens more quickly than if the engine is left just idling.
Second, if this still matters nowadays, pollution is also quite high upon start up with a cold converter, in addition to the fuel wasted for 10 or 15 or however minutes while one is having that morning cup of coffee. In brief, it's a
proposition to me.