my take is that you can't go wrong w/ 0W30 or 0W40, whichever app is "recommended"(in regard to 30 or 40 weight). i have also heard that 0W20 should be avoided except under fully cold climates. obviously one could use it in the winter of moderate climates, and switch to 30 weight the rest of the year.

20 weight, as it was told, was to help manufacturers reach economy requirements by the govt, no?

Hi Bowtie, I agree that you can not go wrong with the weight for hot. But I changed to 0W30 because of the 10 to 0 switch, not because of the 40->30.
0W20: In Europe Honda has now already 2 types of 0W20 on the market (Honda branded). One 'regular' and one especially for their hybrids. The hybrid one is even a bit thinner than the regular! They don't try to 'hide' or disapprove this oil in Europe. Of course, the fuel consumption advantage you have with 0W20 is important for the FE-ratings.
You can not use 0W20 in engines that are not approved to use it (tolerances, especially of the oil pump are main factors to keep the flow and pressure high enough in all circumstances).
But it's an tendency that can not be stopped. 10-15 years ago 0W30 was kind of 'exotic', now it's common.

I've heard this, and may be part of the reason, but Ford has back certified many of their vehicles/engines for 20 weight. Doing so wouldn't affect new cars reaching requirements.

Of course, it may have been in consideration of dealer overhead and storage space.

I switched from 5w-30 to 0w-30 a few weeks back. When i start the car cold, the RPM jump up about 200 higher than with the thicker stuff, and after the switch the car was noticeably more "zippy" before warmed up.

It's too early to tell anything about fuel economy, but
- The engine sound when cold is much 'smoother' and she seems to run a bit more stable.
- Reaction on throttle when moving of cold the first kilometers is also according to me more agile.

First fill up after new oil
 

Filled up today first time after 0W30. Only 4/5 of this tank was with the 0W30.
Reminder: this is not about the Civic, but the small Renault 1.2 petrol of my wife.
Positive result:
- less consumption than two last fill ups, but now it's freezing
- less consumption than February till September fill ups from last year (! strong, isn't it?)
- same consumption as the period May -> June of this year, but than it was much warmer
We only consumed less than last fill up in specific circumstances: sometimes I used this small car for daily commute before I owned the Civic. Only in these periodes we see less consumption.
3 days ago I also made a grill block for the Renault, but only a 50% to take no risk at all as it's my wife who's driving and I have no diagnostic tool for this car to measure temps.