I've said this in other reviews too, a non-plugin hybrid is not a good option for people who do short trips or do a lot of winter. I think parking on P3 at home hurt too, as a lot of these trips were basically uphill out of the parking. Traffic jam. Idle at traffic light. Traffic jam. In super cold -5 was the best day, the other two were less than 10.
But of the hybrids that suffered in the cold, the Prius was the worst of them - cold hurt it more than the others.
The other thing is the Prius suffered more than other hybrids from "regular" driving. Hypermile = awesome. Half-hypermile = still awesome. Use any more than half throttle at any time in the drive and you're screwed.
Which, as I said in the C-Max review, is why I'd buy that instead.
The review has other points too, but that seems to be the one causing most consternation so thought I'd try to clarify it for ya. 
I'd say a lot of short trips in cold weather AND time for the car to cool off between them. If the trips are relatively close together, like taxi use, then the system doesn't have time to cool off. I'd really have to see the mileage of an equivalent non-hybrid to see if the hybrid mileage was suffering more than a non-hybrid.
And if that's not the case, then I'd like someone to explain better than I have, why the hybrid would get significantly worse mileage than a non-hybrid doing spaced, short trips in very cold weather. If the problem was for real, then there must be an explanation.
I have a theory to offer. This may relate to how a hybrid "idles" to warm itself up. They (at least my Escape Hybrid) does not idle at 750rpm, which is about what I'm used to with non-hybrids. The Hybrid "idles" at around 1500 rpm. It doesn't have a "normal" idle. It can drive on highways at lower rpm than it "idles".
And I can tell from the mileage readout that it just slurps gas while doing so. If all this is being done to keep itself warm, and it won't go into ev mode unless it's warm, then maybe they need a mode where you tell it to idle calmly and forget ev mode.
For instance, I can't get 2 blocks from home before having to wait at a stop light. The Hybrid sits at the light with the motor racing at 1500 rpm, guzzling gas. (By the next light it's warm enough to shut down the engine while stopped.) So I've started to shut it off if it looks like it will be a long wait at the light. I understand the newer Escape Hybrids did this by themselves, but I don't know for sure. And I don't know how the Prius handles this.
The upside from all this is that presumably a Hybrid is less polluting because the catalytic converter etc. is more likely to be doing what it's supposed to do. But I suppose that has to be balanced against the fact it's using more gas to accomplish that.
