I think it would be more fair to compare the Fire to the Nook and Kobo Vox. I recently purchased the Vox, and spec-wise it's very similar to the Nook, similar dimensions, bright screen, 8GB internal storage but an SD slot to expand up to 32GB for a total of 40. It doesn't so far allow you access to the Android Market, but it's similar enough to the Nook that there should be a cyanogenmod port at some point.
My feeling about the Vox, and probably the others as the hardware is similarly spec'd, is not a great tablet, but perfect for consuming most media types. Even at 800MHz, it "can" play video at 720p with minimal stuttering, and the battery life is good if you mind it properly. The screen is a bit laggy, and the touch-sensitivity isn't stellar, but I just got it as a way to get familiar with Android, and if I brick it, all the e-readers at this price point are around $200 so no great loss.
The iPad2 is going to obviously kill all three of the others in most respects. But if I spent $500+ on a tablet, and it didn't do any ONE thing well, I'd be a little disappointed. At $200, if it does ANYTHING well, .. I just saved $300 or more.
It's fun to play around with. All the $500-ish tablets seem a lot like buying PCs back in the day, when 486s were the latest and greatest. So, I bought a 386 to familiarize myself with the OS, and won't spend $500 or more, until maybe next generation, more powerful, etc.
If you want a neat toy, any of the three would probably suffice, and similarly suck with the same pain points. The only real difference between them is storage, maybe screen quality, and ease of modding/loading the apps you want. I have gotten my corp e-mail working on it using Touchdown, VMware View client was decent (for very occasional emergency desktop access) and caused some head-scratching showing people a Windows desktop running on a droid-based e-reader. Boxee remote works fine, and I watched about an hour of "The Art of Flight" in 720p on a plane last week, and it only took the battery down 10%.
I'd say all three (Nook, Fire, Vox) are pretty much toys, but cool toys. I like the Vox for the screen brightness and expandable storage.