Well, az99, we own a Fusion Hybrid, and it is my everyday driver. I'll try to answer your specific questions:
1. Do they really get the advertised fuel mileage in mountainous and hilly terrain?
Well, we haven't really driven ours enough in mountainous areas enough to say, but I will say, in everyday driving, it most certainly DOES get the advertised mileage and then some for us! Especially once you get the hang of driving it. I routinely average 49-50 mpg driving our Fusion hybrid to work and back (26 mile commute, 'bout 80% highway).
I will say, though, that hybrid mileage can vary a lot, based on driving style, probably more than a ocnventional vehicle. You drive like a leadfoot and insist on being the first away from the stoplight and the last to start braking to stop, you will pay for that in fuel mileage, and will never see the advertised mileage out of a hybrid.
2. Do they have the same power as a gas only model?
..Yes, and then some. When you get on the accelerator real hard with a hybrid, it will kick in both the gas engine AND the electric motor, giving you a huge boost of power when you need it. I've scratched rubber on our Fusion more than once accidentally, when I got on it too hard. There again, though, driving like that means you won't see the advertised 47 mpg. :)
3. Do they need to be plugged in at night?
No. Hybrids get their charging power from re-generative braking, and from charging that takes place when you are running in gas mode.
However, Ford did come out with a Fusion 'Energy' version of the hybrid that came with a larger battery and the ability to plug it in to charge up the battery. It allows you to run in EV mode even more. But, that model also comes with a much higher price tag. Not sure its worth it, IMO.
4. How far do they go on a battery charge?
Not very far, they're not intended to go long distances on elecric power alone. They're intended to run on electric power when they can and when its most economical to do so, and in gas mode the rest of the time.
On interstates, highways, mine typically runs about 50% of the time in electric mode and 50% in gas mode. It will run on electric power for 3 or 4 miles, at which point the battery runs down enough that it goes into gas mode. Gas engine will run for 3 or 4 miles, at which point battery is charged back up, and it goes back to EV (electric vehicle) mode.
Terrain will affect how much it runs in gas or EV mode on the highway, also. The more downhill and flat roads you have, the more it will stay in EV mode. Conversely, more uphill means more running in gas mode, because the battery and electric motor aren't powerful enough to sustain highway speeds up much of an incline. Thats what the gas engine is for.
When you're off the highway, not going quite as fast and stopping more, you will typically run in EV mode a lot more, about 70 to 80% of the time or more. Since you're using the brakes more, the batteries get more charge from re-generative braking so you can run in EV mode more. Thats why hybrids are known to get such great mileage in the city.
If you aren't watching the instrument panel, you'd barely know when its in gas or electric mode. The transition between the two modes is so seamless, and car is amazingly quiet either way, especially in EV mode.
5. Battery life and replacement cost?
With the new (2013) Fusion hybrid, Ford claims the Lithium-Ion batteries are designed to last for about 150,000 miles. And, they back that up with an 8 year, 100,000 mile warranty on ALL hybrid components - eCVT transmission, motor, and the batteries. I've heard anywhere from $1500 to $3000 to replace the high voltage battery when it goes, but with 8 years and 100,000 mile warranty on it, I'm not that concerned about it.
Anyway, hope this answers your questions. I will tell you that we could not be happier with our Fusion hybrid. The mileage is truly amazing, and its really cool how it can run totally silent and emission free on electrical power so much of the time. Its almost like driving a big, comfortable golf cart, haha. Makes a really great toad also, very simple and easy - just put it in neutral and go. No crazy procedures to go through, no need to run the engine every few hours, none of that. Really nice to have at campgrounds, too, since you can ride around very silently and without putting out any exhaust fumes.
47 miles per gallon, a blast to drive, excellent warranty on the high $$ parts you'd worry about, and easy, simple factory supported 4 down towing....You just can't beat it, IMHO. :)
