Display
The Verge: "Colors aren’t as vibrant or appealing [as the iPad Air 2], the screen isn’t laminated to the glass as on the iPad Air 2, and the backlight bleeds into the edges of the screen in unsightly ways."
Android Police: "it is very, very good... backlight bleed [is] only a tiny bit around the lower-right edge of the tablet, but it's not bad enough to really matter"
Phone Arena: "the Nexus 9 might appear ordinary from a cursory look, but beyond that, it packs all of the rich essentials in giving it some strong visual qualities that we appreciate."
Engadget: "All told, the Nexus 9's LCD is really a good screen, just not an outstanding one. "
Ars Technica: "Other than a lead in brightness, nothing made HTC's 8.9-inch screen look particularly better or worse than Apple's latest tablet."
Conclusion: the backlight bleed is either unsightly or not bad enough to matter, the screen is either worse than the iPad Air 2 or very similar but brighter, and overall it's either great or mediocre.
Sound
The Verge: "I wish every smartphone and tablet had the Nexus 9's speakers"
Android Police: "The quality of the audio, though, isn't exactly mindblowing. Mids sound kind of muddy and treble performance isn't stellar... I'd guess these are the same speakers in the One M8"
Phone Arena: "it still reverberates with a deep punch. Listening intently to the quality, it doesn’t crackle or strain at the loudest volume, but it sounds echoey at times."
Engadget: "more subdued and muddled than I'd hoped. Now, lest you think I'm being harsh, the speakers aren't bad by any stretch... not as loud as the M8 [and] the crisp channel separation you'd get out of the M8's BoomSound speakers is missing here, "
Ars Technica: "The result sounded pretty horrible, and I never found a convincing case where the Nexus 9 played TV or movie audio in a way that justified its equalizer, which can't be adjusted or disabled."
Conclusion: either the same as or worse than the M8, either good or bad bass, possibly the best tablet speakers but possibly awful.
Performance
The Verge: "Stutters and slowdowns are plentiful, apps take a long time to open and load"
Android Police: "In terms of raw speed, I would not say the Nexus 9 is slow. The problem is more that it jumps and pauses when doing certain things, and it makes for a jarring user experience in certain situations. [Compared to the Nexus 7 the] Nexus 9 is significantly quicker if an app is already in memory, and the launcher definitely felt a bit snappier"
Phone Arena: "Without question, the Nexus 9 screams with its operations – one that rarely exhibits any sort of delayed response or choppiness. Normal, baseline tasks are all handled with tight responses, but the Nexus 9 delivers great handling with mobile gaming as well. Obviously, the benchmark scores give us a telling tale that it’s a beast."
Engadget: "the Nexus 9 can and will handle just about anything you throw at it. As I made abundantly clear in the software section, the Nexus 9 runs incredibly smoothly while you poke around the OS and launch apps."
Ars Technica: "However, I noticed a fair number of stutters when jumping from task to task, like when starting a new Chrome instance or quitting out of a high-intensity app and waiting for menus and icons to pop back on the screen. "
Conclusion: either very fast or very slow.
Battery
The Verge: "Battery life from the Nexus 9’s 6,700mAh cell is closer to the iPad mini than it is to the iPad Air. It will get through a full day of heavy usage, two or more with light usage, which is probably fine for most people."
Android Police: "I might have been able to edge out four and a half, maybe five hours of screen-on at the rate I was going (35% remaining with 3h15m screen on), but I still wasn't doing anything crazy demanding of the tablet.. Either way, Google's battery estimates on the Play Store seem to be basically, if I'm blunt, wildly inaccurate. I'm not sure what universe they're getting 9.5 hours of 'Wi-Fi browsing' in. It's not this one."
Phone Arena: "Even better, though, is how it performs in our very own standardized battery benchmark test. Producing a mark of 9 hours and 24 minutes, that nearly matches Google’s claim of 9.5 hours of juice with Wi-Fi browsing. "
Engadget: "It was nothing if not an able companion as I plowed through my daily routine, sticking with me through about 12 hours of mixed usage... The first few times through our standard video rundown test (with an HD video set to loop indefinitely while screen brightness is locked at 50 percent), the Nexus 9 usually managed to hang in there for about 9 hours and 10 minutes before giving up the ghost. "
Ars Technica: "Our usual 200-nit battery life test saw the Nexus 9 hit the same eight-plus hour mark as other recent devices, and the device didn't experience other significant drain from normal day-to-day use. "
Conclusion: who the **** knows. Android Police could probably do with actually running it down with standard usage, instead of running a bunch of benchmarks and then estimating from partial discharges. Only Ars specify a normalised brightness and run a benchmark.
Overall Conclusion: the ridiculous inconsistency makes it pretty difficult to review these websites, although Ars Technica seem to make relatively sensible and evidenced claims (and tally up well with Anandtech's preliminary findings). My advice is to wait for the Anandtech review or until you can borrow a friend's for a while, because if you asked this lot the time of day you'd get four different answers and the fifth would start rambling about the Air 2's clock app.