i've just dropped off my IS250 to Lexus of Brighton (where I purchased the car) for a 45,000km service.
Lexus of Blackburn actually appears to be the most price competitive when it comes to servicing costs. According to the fixed price service, a 45,000km service for the IS250 is listed at $585.
Lexus of Brighton had my service listed at $680 on the service sheet.
I've visited Melbourne City Lexus to purchase parts (spoiler and wheels) and have found them to be great in service.
I haven't had any personal experience with Lexus of Blackburn.
Lexus of Brighton have always been good but do appear to be the most expensive in service costs.