For me it seemed like a no-brainer... same performance, and less fossil fuel. I believe the 40i is rated at 23mpg while the 45e is 20mpg, without electric assist. Assuming at least 26 miles of pure electric driving, you should be better off in the 45e for any trip less than 200 miles, at a minimum. That would be literally running it electric until a dead battery and then all gas for the remaining 174 miles. But with the hybrid and navigation calculating the best combination of gas to battery, it should provide even better efficiencies. I would imagine that the 45e with a full charge, would be more efficient for a full tank of gas. The problem is when you can't recharge on the return trip. But in those limited cases, the shorter trips should more than make up for it over the life of the 45e. I'm not factoring electric cost, as that isn't important to me. If I can afford this car, I can afford the cost to run it. What is more important is reducing fossil fuel consumption and engine pollution. I feel like stop and go traffic would be really pleasant in a full electric mode, an added benefit. The only downside to me, was the loss of some storage space. I guess there is some weight performance hit, but I'm not looking to hit the track in it. If they had included fast charging it would have been the perfect car, but then BMW needs to keep you on the hook for the next improved model
I don't take delivery until the end of April, so none of this is first hand experience.