A couple of things you can do for better gas mileage is put in a better air filter and buy something called a condensator ( http://www.condensatorsales.com/ )
With these two additions ive gotten an average of 22/28 on my sc300, which usually gets 18/24
As far as reliability, the vehicle will easily outlast any German or American vehicle on the road - the idea behind lexus is that toyota wanted to create vehicles that could compete with other luxury vehicles in the performance and comfort field and remain as reliable as the rest of the toyota lineup. In this task, they have succeeded - as they have won many awards for being the most reliable vehicles on the road.
But when repairs are needed, which doesnt happen all that often, the lexus dealer will rape you for everything your worth. Fortunately however, you can usually take your vehicle to a toyota dealership and they will be able to repair it, or you can get onto a forum (such as this one) and ask about your problem then fix it yourself. So, in the end, the vehicle will last more than 200k miles and you can almost certainly get it repaired with minimal damage to your wallet.