Just traded my LS430 in for a GS so can give you my thoughts (it's for sale from Bracknell if your interested :) )
Economy - short journeys I got around 17 mpg but if you do longer journeys 28-30 mpg was common on a long motorway journey. It's an 85 litre tank so range is huge.
Servicing every 10K - minor service was about £190 and I think major service is about £350.
Tyres - I got 25K out of the rears before replacement.
Generally bulletproof quality but be aware that the LS has LOTS of electrical toys so check them all (seats, mirrors, sun roof, self closing doors, steering wheel etc). It is also worth getting a new battery fitted if you buy one.
Also be aware that the LS is a heavy automatic so get brake pads and discs checked.
Only problems I cam across during my time with LS was a sagging drivers seat (many are used as chauffeur cars) and mine went through a few front parking sensors. All of this covered by warranty.
Overall, a great car that you won't be disappointed with!
Good luck