If your work involves a lot of writing, then the iPad isn't an obvious boon [1]. But if you can do your work more efficiently by treating data and actions as symbols, the iPad is just overflowing with potential, much of it already being realized. Musicians, artists, animators, logicians, teachers, doctors, researchers; the list of professions offered new ways to work is getting longer and longer. To these people, the iPad can be life-changing.
I don't think the Kindle Fire will change anything. It's just not in Amazon's DNA to make a product focused on anything other than buying and consuming content — and there's never been a shortage of ways to buy things.
[1] http://arstechnica.com/apple/news/2011/09/doable-or-not-my-e...