My experience with Zero since 2011 has been the following:
Model Year-2010
Model-DS
Purchase date (mo/yr)-June 2011
Mileage-5200km
Modifications? If so, what?- None
Primary terrain during use (city, highway, dirt, combo, etc)-City
Max/min/average ambient temperatures during use- 45/0/20 degrees Celsius
General opinion: I do not recommend 2010-2011 bikes...
Model Year-2012
Model-S
Purchase date (mo/yr)-June 2013
Mileage-16000km
Modifications? If so, what?- Topcase rack, front windshield, handguards, plastic motor cover.
Primary terrain during use (city, highway, dirt, combo, etc)-City
Max/min/average ambient temperatures during use- 45/0/20 degrees Celsius
General opinion: Nice bike but still not reliable enough in stock condition. With the BMS and motor change a very nice bike.
Model Year-2014
Model-S 11.4
Purchase date (mo/yr)-March 2016
Mileage-1300km
Modifications? If so, what?- Original topcase rack
Primary terrain during use (city, highway, dirt, combo, etc)-Combo
Max/min/average ambient temperatures during use- 45/0/20 degrees Celsius
From my point of view the 2012 models were a big improvement over 2010 models but still had some known design flaws (mainly related to environmental resistance of the BMS and the motor).
I don´t have enough experience with my 2014 yet to give an opinion with regards to reliability, but the general feeling is that the bike is better engineered than the previous models and the performance is way better. There are a couple of things which could be better (the wiring below the seat seems too exposed and the motor cooling is not good enough) and from what I´ve read here the chargers are not very reliable... Neither does the rear shock absorber that seems to be the same used in 2012 and 2013 models.