I noticed a slight drop in FE (in my AWD FEH) as the weather got cooler, dropping from average daytime highs of 60+ down to the lower 30s. The difference was maybe a couple of MPG (say, from 32 down to 30). Then when it got REALLY cold (for us), with daytime highs only in the teens, there was more of a drop, down to maybe 27 mpg. The FEH does seem to act a bit differently when it is VERY cold - the ICE (internal combustion engine) doesn't shut off as often, and comes on again after being off for just a short time. It seems as if the batteries aren't holding as much charge (I've heard this is an effect of cold), perhaps only 1/2 as much, so they discharge more quickly and the ICE has to come on. ALso, it has to come on more to keep the catalytic converter (and perhaps some other parts, I'm not a motorhead) warm enough. And the engine seems to race faster under load, but that might be my imagination; when it's that cold my brain freezes and doesn't operate very well!

So far I haven't had the Prius in any extreme cold (below about 25), so I don't know how it will behave under those conditions. It does pretty well in moderate cold, daytime highs have been in the 30s - 40s, with some rain and slush, and I'm getting about 47.5 mpg so far on this tank.

You will definitely see lower mpgs in cooler temps, but still probably substantially better than a non-hybrid, similar vehicle (at least that's true of the FEH, our 4WD V6 non-hybrid FEH gets only about 17 mpg in the winter, so 27 is still better than a 50% improvement).

Also, with the hybrids (and their "instant" and "average" FE meters), it's more "painful" (at least if you get the hypermiler mentality) to leave the car idling to warm up the inside, defrost the windshields, etc., so it tends to feel like a colder ride (at least for the first 5 minutes). And the defroster itself (in the FEH) keep the ICE from shutting off at all, so that would decrease mpgs even more if you had to keep the front defroster running. Of course, that's only a factor when the ICE would otherwise cut off (at stops, and during low-speed driving) - if you're doing 55+ on the highway with no stops, the ICE will be running anyway.