I've had the iPad 3, Nexus 7, Kindle Fire HD 8.9 and am currently using a Kindle Fire HD (7.0).
The iPad is a great tablet and I'm sure the iPad 4 is nothing short of awesome. And the tablet apps on iOS really make this tablet shine. I just sold my iPad 3 and maybe I'll get an iPad 4 later. I'm undecided. I'm starting to find that I really prefer a smaller size. And really a lot of great options for under $500. So I'm still weight if the apps I love on the iPad are really worth it.
The Kindle Fire HD 8.9 kinda disappointed me. The Kindle UI felt smoother and quicker on the faster 8.9 inch model than the smaller 7 inch model. But after I rooted it and side loaded some apps, I noticed lots of small issues. Chrome is pretty sluggish, Dolphin for some odd reason jumps to the bottom of new pages, Nova and Apex launchers flip upside down when trying to move or add icons in landscape mode. And oddly apparently the 8.9 inch speakers are no dual driver, they sound good, but aren't as loud as the 7.0 model.
I had the Fire HD 7.0 briefly before getting the Fire HD 8.9. I returned that smaller Fire HD because I thought the 8.9 was going to be a better experience. But I ended up returning that 8.9 inch and buying the 7.0 inch again. I rooted it, side loaded Google Play and other apps. Chrome is so much smoother, Dolphin works perfectly, Nova and Apex launchers work flawlessly (along with other launchers). The Kindle UI animations are noticeably slower, but it seems like the apps I side loaded are faster and have pretty much no issues. The speakers are louder. And I'm realizing I really like the smaller 7 inch size more anyway. A big reason I bought both Kindle Fire HDs is because I use a lot of Amazon content, including reading ebooks. And reading books is much better on a 7 inch screen I can comfortably hold in one hand.
I was really disappointed with the build quality of the Nexus 7. I went through 4 or 5 of those dang tablets, trying to get one that didn't have loose glass on the left side. I hope they have that manufacturing issue solved by now. The color quality of the Nexus 7 is also disappointing, screen looks washed out, greens and yellows look too pastel, white saturation isn't good. I had my Nexus 7 for about 2 months praying Google would be able to fix the screen calibration with an update, but that never happened. The speaker also really sucked. For $200 it's nice and for someone that isn't into Amazon content I'd say it's a much better choice than a Kindle Fire HD. But I like my Kindle Fire HD because of the better looking screen, much better speakers, and easy access to all my Amazon content, and I also like the suggestions and ease of purchasing new content and the Lending Library (X-Ray is kinda okay too).
It's a shame the iPad mini has such disappointing specs. I'm sure it works great, but just hard to pay $330 for something with a 1024x768 screen and a 2 year old SoC. I might need to see this in person to see if the lightness and thinness can't change my mind or something.