First, to those getting poor mileage I would ask have they driven over 5,000 miles and through a full year? Typically, hybrids have a 3,000 mile or so break-in period where the mileage tends to be well below the expected long-term average. Also, mileage tends to be best when the weather is cool enough not to need A/C and warm enough for the engine to shut down. Currently hybrids rely on the exhaust to heat the catalytic converter, in the winter this means added idling or a richer fuel mix. Hopefully some auto manufacturer will fix this issue in the future.
Also, I would ask if the tires are under-inflated, do they tend to speed (especially over 70mph), have they modified the vehicle in any way (different brand of tires, roof rack, etc) or is there a lot of weight in the vehicle. All of these lower your mileage.
Also, consumers should be forewarned, at highway speeds wind resistance works very much against you and hybrid technology cant overcome poor aerodynamics expect the new hybrid Trucks/SUVs to be particularly thirsty on the highway.
As for myself, I commute 60 miles a day and traded in a 25mpg Ford for a 59mpg Insight (with automatic transmission EPA is 56 Hwy/59 City). The computer tracks the vehicle's lifetime mileage, and after 60,000 trouble-free miles Im averaging about 61mpg. Yes, in fact it isnt too difficult to BEAT the EPA estimates once you learn to drive for mileage.
If I run the numbers, Ive saved $2125 assuming an average price of $1.50 per gallon since switching vehicles. Thats more than enough to cover the premium on the hybrid. Plus with gas prices as high as they are now its a nice feeling not to have to stop so often for fuel.
As far as reliability, the verdict is still out, though by now many early adopters have passed 100,000 and 200,000 miles, so reliability looks promising. Additionally, electric motors tend to wear better than internal combustion engines (ICE) because of fewer moving parts and less heat being generated. Hybrids, while having two systems instead of one, do allow the ICE to work less hard and may turn out to be at least as reliable as a standard drive train.
As for Diesels, they do get good mileage, but produce much more carcinogenic soot than any other type of engine. Also, expect diesel to become much more expensive as new fuel blends to reduce some of this pollution become required. And the availability, odor, etc of diesel fuel do impact the ownership experience. Ive personally owned a diesel myself and wouldnt go back to owning one. Especially with gas-electric hybrids being an option.
