I've had a 2012 Nexus 7 Wi-Fi since early 2013. It came with 4.1.2 JZO54K Jellybean. I've loved the size and the access to the latest version of Android. I dropped it late in 2013 and replaced the screen myself.
The Android journey has been long. Along he way I upgraded it from 4.1.2 to 4.2.2, 4.3, 4.4, 4.4.2, 4.4.3, 4.4.4, 5.0 5.0.2, 5.1.0, and 5.1.1.
This was hard on the Nexus 7. The Internet is full of bad experiences of updating the 2012 Nexus 7 to Lollipop. My experience was pretty much aligned with these.
The Nexus 7 would just crawl. It especially seemed to be slow if you tried to do anything while you were letting the Google Play Store update apps. Once it got slow there was no fixing it. Rebooting didn't help.
But after my experience with adb on my Nexus 5 I got brave enough to reflash the Nexus 7 with 5.1.1 LMY47V.
That process was similarly complicated as the Nexus 5 process. At one point the screen displayed "REBOOT FAILURE." And when it finally came up it was sluggish for several hours.
But the result was well worth it. The Nexus 7 is working fine. Sure it's not as fast as a new Android tablet but its performance is consistently Ok. Not bad for an almost 4 year old tablet.
I left the bootloader unlocked so I'm watching for CyanogenMod to drop Marshmallow.