A sensorless ESC will ALWAYS 'cog' to a certain degree... better sensorless speed controls however will cog less but a sensored speed controller will always be better off the line (so long as its working correctly)
All brushless speed controllers need to know where the rota inside is in relation to the three coils inside the motor in order to maintain control of it. A sensored motor does this with (ahem) a sensor - from pretty much standing still the ESC knows where the rota is inside hence a smooth pull-away - a sensorless ESC on the other hand uses the 'back EMF' from the coils inside created by the rota (permanent magnet) spinning to know where it is... to get the motor spinning from a standstill the ESC fires the coils in a set sequence to 'force' the motor to spin - it takes a little while for the speedo to work out where it is (hence the 'cogging') - once its up & running the control should be smooth.... there is an argument that a sensorless motor/esc combo has the potential to be faster... but at the expense of off-the-line smoothness.
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