I used an OEM one in my car

disadantages of not running a thermo
1) engine doesn't get up to operating temp (increased wear & tear)
2) when it doesn't get into op temp, the ECU might stay in cold start mode - increased fuel consumption
3) When a thermo stat is open, it blocks off the coolant bypass - without a thermostat, and if you don't block off the coolant by pass, the coolant may not go into the radiator anyway, so you overheat.