I was in a similar position to yourself, I had the IS300 which I had for about 6 years, no problems, fantastic car. I replaced it in June 2010 for a GS450h SE-L (07 reg) which is also a lovely car to look at, drive etc, however, in the past year the car has been recalled for replacement of valve springs, with no further problems. The left cylinder bank cam chain tensioner disintegrated twice. The car is now running as it should and is very quiet and comfortable at any speed.
To elaborate on the cam chain tensioner problem, I have no idea why it occured the first time but I am pretty sure that the second time was caused by the chain being streched during the first episode. The garage was very good with their investigations, but not having a new chain to check the length, and the chain looked OK and appeared to be serviceable (I examined the chain personally while the engine was dismantled). Everything appears to be OK now and the car is again wonderful to drive.
If I really drive carefully, I have achieved 52.6 mpg but back to reality, mid to high 30's is achievable without having to try too hard. mid to high 40's is achievable on longer journeys.
To summarise, now that problems are sorted, I am very pleased with my car.
