Once the Nexus 9 began to ship, I read all the reviews written, and the consensus has been that the hardware build quality did not meet the expectations for a tablet at its $399 price. The reviewers were comparing the Nexus 9 to the Nexus 7, which drew high praise for a combination of features and low ($249) price. In my experience, I have absolutely no problems with the hardware build quality, but I have had problems with Android 5.0 "Lollipop."
Most point out two hardware problems: flexing of the back case when pressing in the middle near the Nexus logo and light bleed along the edges of the screen. Consensus seems to be that these items are not show stoppers, but disappointing for a tablet that costs as much as the Nexus 9.
The back case of my Nexus 9 does not flex when I press on the middle near the logo. Either assembly and/or manufacturing improvements were made before my Nexus 9 was assembled, or people are being really picky. I do see some light bleed, most noticable near the top right of the Nexus 9, but again I really don't see it as a problem. Frankly, I think people are using their disappointment over the price of the Nexus 9 to color their opinion rather than evaluating the device on its merits and how it best fits their needs.
Where I have seen problems using my Nexus 9 is in the software, and I suspect it is due to a combination of bugs in Android 5.0 and apps not being fully compatible with this latest version of Android. The Facebook app particularly seems to have a problem.
Occasionally the screen just goes black when attempting to return to the home screen, although I can see the top status bar and bottom navigation buttons. Sometimes I can resolve the problem by switching to another app, and then selecting home, but there have also been times when I have had to shut down and restart the tablet to get it functioning properly.
Another problem I have had is home screen widgets failing to update, and the only fix is to restart the tablet. I note that both problems relate to the home screen, and seem to me to be memory related. It's possible there is a memory leak issue causing the problems.
The good news with software problems is that they can be fixed, and Google has already released an update to Lollipop, Android 5.0.1. Unfortunately, while 5.0.1 has decreased the frequency that these problems appear, it has not eliminated them.
What cannot be changed are the hardware features, and I can report that I am very happy with the Nexus 9 hardware. The most significant feature of the Nexus 9 for me is the screen size and corresponding aspect ratio, and I love both. Open a book in the Kindle app and what you see is nearly identical the a printed page of most books. Web pages are very readable, and content in Evernote and Pocket display wonderfully.
The Nexus 9 is not as light as the Galaxy Note 8 it is replacing, but is much lighter than my iPad 3. I cannot hold the Nexus 9 in one hand for as long as I do the Note 8, but it is more comfortable to hold in two hands for long periods of time than the iPad. Would I like it to be lighter? Sure, I wouldn't complain if it where lighter, but I find it light enough.
Battery life for the Nexus 9 is not as good as the iPad 3, but given that the Nexus 9 is smaller and lighter than the iPad, I really don't expect it to be. GSam Batttery Monitor showing average screen on time per charge of 5 hours, 20 minutes and most the time battery discharges at a rate of 2 to 3% per hour. I don't think the battery is completely conditioned yet, but I am not having any problem with the battery running out during my normal daily use.
Bottom line is that I am happy with my purchase of the Nexus 9. As I wrote previously, I don't find the iPad Air 2 more compelling and with a larger screen, I prefer the Nexus 9 over the iPad Mini. I say this while fully acknowledging a bias towards Android. While I disagree with other reviewers who think the Nexus 9 is too expensive given their problems with its build quality, I do think that unless a person has a bias between iOS or Android, a decision to purchase the Nexus 9 for most will come down to price.