I am a happy user of one plus one. It is truly a flagship killer! It is regular with system updates and optimization and till now I have had a lag free experience. It is a value for money Phone, has all the top notch specs which a flagship needs to have.
It easily defeats many Phones in the higher price category which is the main reason to buy it. I would definitely recommend going for this phone and if your budget is little higher then go for the one plus two, another great device!