I've have a potential "worst case scenario" setup on my car, with some fairly heavy, not very aerodynamic 16"s (stock is 14" on '03) with tires that are 20mm wider (205 vs 185) and not specified as Low Rolling Resistance. Initially, mileage hit was absolutely huge, as in >20%, although I believe this was mostly due to some unrelated problems with the battery management system that I accidentally "fixed" by draining my 12v battery. The battery then recalibrated itself and performance immediately improved by a noticable margin, and the mileage "hit" was cut in half to 10%. Since then, the tires broke in some more, I adjusted for the fact that the old tires were actually slightly smaller diameter due to all the tread being worn off, and found that actual difference is 6-8%, depending on temperature (though this is still showing signs of improvement). That hit is definitely worth it to me though -- the ride is smoother, the car corners FAR better, and wet handling is leaps and bounds better. The look of the car is also greatly enhaced with the wheels. The stock low rolling resistance tires toward the latter part of their treadlife would begin to slide even on dry terrain, would spin when starting on wet roads, and were downright scary for wet cornering. Even new though, they were still quite poor.

As far as mileage hits go, you'd probably see about the same amount of extra fuel consumption in absolute terms on a gas-only vehicle, and a hybrid (all things equal). However because hybrids are so efficient in general, that will represent a 6-7% hit instead of a 3-4% hit. The same is true of running Air Conditioner -- the same amount of gas burned might be a 12% hit on a hybrid, and a 1% hit on a huge SUV, having that huge MPG display in front of you though makes the whole deal seem a lot worse.