I'm the second owner although had it for more than 3 years with no problems. Although its been regularly serviced by a non-Lexus garage, Lexus are not prepared to offer any good will concessions even though problems with the cylinder head are very remote.
The symptoms are intermittent slight rough running at low speed - this disappears at speeds over 20 mph or so. The rpm doesn't increase but there's slight vibration and roughness.