Our current gas water heater is 7 years old and now past its warrenty. I'm not sure how long they are designed to last. Depending on the brand the average range is 1 to 10 years. Some last 15-20, but those seem to be rare exceptions.
The benefits of the electric tankless :
1. Our electricity is solar/wind, so now our hot water would be too(instead of natural gas).
2. This model is 30-40% more efficient than a gas tank model(which is what we have).
3. With just one bathroom, we can't draw enough hot water to overpower this thing. You could take a 15 hour hot water shower. Especially when the in-laws visit, we don't have to worry about using up all the water. Even with just Emily and I, I tend to wait a bit after her shower to give the tank time to refill.
4. It is small(16" by 17" by 3") and only 12 pounds. We could unplug it and move it with us to a new house(if we ever got one).