We have a kindle and an iPad. The ipad is great for reading in the dark as it is backlit, the iPad sucks for reading in the sun for the same reason. So I do enjoy them both. The bateery life of the iPad, and I imagine the kindle fire, will be nowhere near the battery life of the regualr Kindle as it has to support so many more things.
We were contemplating getting a Fire as a cheaper alternative to another iPad for hubby to use on his travels, but I think in the end we have decided to pick up an Acer tablet for $300. Reasons are, Kindle Fire is not sold or supported in Canada. The old Kindle is an awesome e-reader and the battery life is about three weeks. The KIndle Fire is not going to be as powerful as the Acer tablet, and for internet only applications, it will do the job he needs it to do and he will use the kndle to read. The Acer tablet also has an SD card slot, nice touch for photos etc.
Both our kindle and iPad use WIFI, it has never been an issue, I can tether the iPad using my iPhone or I can pick up free WIFI in stores or hotels, most schools and convention centres have it too.
SO the answer is no, we won't be getting a Kindle Fire, though is does look FUN!