I get much more use out of my 9" HD+ than I did from my previous 7" Kindle Fire and Nexus 7. However, size, weight and general form factor are very important in larger screen tablets. I haven't had my hands on the new Fire HDX 8.9 but it's a bit narrower and considerably lighter weight than the previous model, 374g vs 567g.
For comparison, the iPad Air weighs 469g and the Nexus 10 comes in at 603g. The B&N Nook 9" HD+ weighs 515g. The 2nd Gen iPad was a hefty 680g. Among the 7" models, The 1st Gen KF weighed 413g and the original Nexus 7 was 340g. The new 7" Fire HDX weighs 303g and the Nexus 7 (2013) is 290. The iPad Mini 2 weighs 331g.
I do most of my reading on my e-ink Kindles (PW and Basic) and they're certainly much lighter and more comfortable to use for extended reading sessions, but I find my HD+ to be surprisingly comfortable and frequently use it for hours at a time for watching videos and reading graphic novels. I did find that adding a folio-style cover made it feel heavier and more cumbersome so I switched to this silicone case:
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1 and it's perfect. I also have a zippered sleeve for added protection when carrying it around but don't use it much.