I am really surprised to see that most comments say that price is the main difference and that the Hikvision products are virtually identical to Dahua and UNV. I think this is not correct. While there are alot of similarities there are certainly many differences between Hikvision, Dahua and UNV.
Hikvision has Darkfighter and Lightfigher, Dahua do High WDR and Starlight in the one camera.
Hikvision have the Panovu cameras, Dahua and UNV do not
Dahua makes great Video Intercom systems, Hikvison is not as good, UNV does not have them
Hikvision has TVI max of 5mp, Dahua has CVI max of 4mp, UNV has not HD over Coax at the moment.
Hikvision and Dahua have intelligent parking solutions, UNV do not
UNV has the cloud firmware update feature, Hikvision and Dahua do not.
Dahua and Hikvision POE in NVR can do 100m and UNV can do up to 300m
Dahua recorders generally have better decoding performance than Hikvison and UNV.
Hikvision have a nice little recessed camera ceiling mount camera. DS-2CD2E10F
I could continue on with a big list of the differences however my point is this. Hikvision, Dahua and UNV are not the same products. Once you know them better you will start to understand the differences between them.
Overall i think it is a bad idea to say things like, I only use Hikvision or I only use Dahua. You should be selecting the equipment based on project requirements.
If you need log runs out to the cameras use UNV because Ip cameras can be up to 300m away from the POE recorder.
If you are running alot of high resolution cameras Dahua might be good because for a similar price point the NVR can decode more high resolution cameras than UNV and Hikvision.
If you need a good looking camera for an office environment go for the DS-2CD2E10F from Hikvision with a Hikvision recorder.
As an integrator you should simply be choosing the right product for the project.