I was one of the "dreaming of a Nexus 10 update" crowd. The idea of an improved N10 had me drooling, which itself says how I feel about the original. When the time came, bearing in mind that I am not an early adopter, I had to fight the urge to pre-order a Nexus 9.
I am very glad that I didn't. The customer reviews are very mixed, with Amazon complaints mounting to the point where they actually issued an official company review of the product (that means they pull it and see what's up - currently, buying it on Amazon means buying from the alt marketplace).
I'm taking the time to sit down and really decide if my N10 has aged, or if I'm just like most of the masses, a.k.a. feeling pushed to have the latest-n-greatest while making up superficial excuses to justify it. In short, I think the N10 holds up enough to hold onto it for at least another year. Is it paper thin? No. Is the screen the best out there? Outstanding by 2012 standards, but now it's just really good. Does the battery last all day and night? No. I wish it latest longer. Is it a powerhouse? Again, it was a beast in 2012, but now it's a bit behind the curve. But it runs buttery smooth, for me at least. I can blast through all of my apps and menus with only rare stuttering (even the brand-spanking-new N9 isn't immune to that). And it's getting Lollipop. That might even help.
TL;DR - I am keeping my N10 and holding out for Lollipop. I am glad I didn't get an N9. If you can find a new(ish) one for cheap, I say go for it. Heck, even the 2013 N7 would be a good buy, and it would be even cheaper. It's also getting Lollipop.