I've been using my Kindle Fire since it came out, and while I'm still waiting for CyanogenMod9 to come out and let me actually put Ice Cream Sandwich on my Kindle Fire, I've been relatively happy with the performance.
The main interface tweak added to the Kindle Fire is the ability to remove items from the carousel on the home page. This is a small but useful way to keep your most commonly used apps organized and, if necessary, make sure other users don't see whatever naughty things you might have been perusing.
That's the only change to the main screen; you still can't organize your apps into categories or customize your menu beyond adding and removing items from favorites and the carousel. I use my Kindle Fire for several different things, and it would be great to organize my apps by categories like Online Content, Books, Network Tools, and Games. The Fire still has Amazon's default seven tabs and single app list organized alphabetically or by date.