Same dilemma here as dd is 10 next month and wants one. Looking at kindle fire at £159 - good apps but not much memory, and Blackberry playbook which is about £130 with lots of memory but not so sure about their app store and never used blackberry!
Blackberry has terrible support and a below par app store. The Kindle Fire HD is fine as a device, but you are locked to Amazon's app store which does not have everything that Google Play offers.
If you are considering the Kindle Fire HD then you may as well go for the Nexus 7 if you can stretch a couple more pounds. It's a far better device, it runs on pure Android and has full access to Google Play. IMO there's no single good reason to buy the Kindle Fire HD over the Nexus 7.