There are a few threads around about it already, try using the search. like http://au.lexusownersclub.com/forums/topic/10006-75k-service-due-soon/page-2 But things to consider, are the local shops using genuine parts, do you get a loan car, are they doing it properly. Ultimately, its up to what you are willing to pay. I did a service at ultratune once, they spent the whole day doing spark plugs and didnt do anything else. Waste of time.
Of course every ultratune is different but I'll go to lexus for the major service for peace of mind.