I haven't completely written off the Nexus 9 just yet. Not by a long shot. It's a Nexus tablet, and that, by definition, means it's important. It's also the first truly 64-bit platform we've got, and so it's important for developers as well.
The real question is what's going to compete with it? Or, rather, what already competes with it?
If you've got to have something in a wider aspect ratio and smaller size, the 2013 Nexus 7 (if you can get one) is still a really good option. Samsung (as Samsung does) has a number of excellent alternatives. The LG G Pad 8.3 has been underrated. ASUS has a number of other offerings, too. And don't forget Sony.
And those are just the Android tablets we have today. More are coming, and soon.
For now? You're not crazy if you hold off on buying a Nexus 9. And neither are you crazy if you spend your hard-earned $399 on one. (Though we'd recommend splurging $80 more for the 32-gigabyte model, if you can — and the "sand" color is gorgeous.)
The Nexus 9 has growing pains, is all. But let's hope it grows out of them sooner rather than later."