I have a 2012 Nexus 7 and I'm wary of people saying they didn't have performance problems until Lollipop. If I was using it for much more than an e-reader the lag would have forced me to upgrade a year ago. I don't question the other issues reported but I haven't experienced any of them. The only thing I don't like so far is the combined notification bar that requires two swipes to reach the menu that used to only require one.
Upgraded my wife's 2012 Nexus 7 when she let me pry it away for the 10 minutes needed. It's working just fine she reports - smoother in general.
On the other hand, Lollipop gave new life to my 2012 Nexus 4. Battery lasts longer on tether, the animations ease load times.
There won't be any more major Android releases for the Nexus 4, but now I think it would last nicely until the next Nexus/ iPhone S appears, which is likely to be a year from now.
I haven't seen any issues with my nexus 5. Would be nice to know what devices they are experiencing errors with.