The ECO button
 

Is there a need to use it in the winter?

It's getting cold here in the Northeast, and since I have the temp set to 70, I would think the AC compressor shouldn't be engaged. If in fact that's the case, would the ECO button even do anything.....or is the compressor always engaged, perhaps to keep the interior humidity down or something?

You want the compressor to come on when defrosting to prevent/clear condensation on the inside of the windscreen.

Correct, but:

1) The air is already pretty dry here in the winter, and

2) I don't think I'd want the compressor to run continuously, which would result in some loss of power, increased gas consumption, and wear and tear on the compressor.

Actually, don't touch that button. It's good for an a/c to run, even in the winter, for it keeps the seals lubricated, and will reduce the need to service it in the spring. Today's compressors are so well designed that they barely drag power off the engine, and besides, you really have plenty to spare.

wow you guys have eco on these cars?
my AMG has one too

I recall reading in the manual that the A/C doesn't come on if the outside temp is below x degrees. I believe it was 35 degrees. I'm in the NE as well, so i was concerned, but now i don't really use the ECO button.

I wish it could turn off the A/C (by pushing the ECO button), but keep the ventilation in Auto mode when the outside temp is between 35 and around 55. Does anyone know how to do that? It seems like an overly luxury mode to have the A/C on almost all the time.

Does anyone know when the ECO button was changed to AC OFF?

In the US 2007 and earlier ECO - 2008 and later AC.

I think the air smells better when the compressor is active. I always get the feeling that the air is slightly stale when using ECO, or "unconditioned".

I drive my 2007 C2S all the time in the ECO mode. When the outside temp allows, I prefer it in the vent mode. There is no issue with 'keeping the seals lubricated' if the AC is used periodically.

I usually turn on ECO(AC off) a mile or two before getting home while the fan is still running. I have been told it helps with mildew build up in the HVAC system.