Although I've had Lexus GS before, I'm only halfway through my process of buying a GS450h myself. However, here's some thoughts from the viewings I've done so far.
1) Cars aren't selling quickly. I've seen and test driven five (main dealer) cars over the last eight weeks, and every one of them is still for sale, as of today. And I'm pretty sure that other cars are still stuck on forecourts. To state the obvious, large engined executive cars don't sell quickly. I'm pretty sure that no private cars have been successfully sold - they just sit there, at the same asking price. (For some reason, Lexus private sellers think they can ask dealer prices, which is insane because they can't offer the Lexus warranty, nor the cheap financing that everyone seems to want these days)
2) The new 450h starts being delivered next month. Inevitably many will be sold to existing 450h users, which will result on more used cars hitting the used market in July.
3) Compared to Mercedes and BMW, GS450h cars ALL seem to have diligent service records. So main dealer history is the norm, not the exception. For BMW and Mercedes, users generally stop using main dealers once the warranty expires.
4) Creaking interiors are a cause of irritation amongst existing users. Part of this must be because of the overall quietness of the drive-train and suspension. But it's clear that some cars do have this problem. The dashboard is the usual problem area, but leather interiors are another problem - I rejected one car just because of this.
5) Ride quality varies. I don't know why, it might be tyre wear or suspension set up, it might have been the roads I've used. But there are differences.
6) Despite what you might expect, the hybrid drive train doesn't seem to be any problem at all. There's no discussion of problems. (The one exception might be occasional jerkiness, but that's a characteristic of a new car, not a used problem). Oh, and to state the obvious perhaps, these cars are incredibly reliable overall compared to their competition.
7) Fuel economy is fantastic for the power of the car, but not for the size of the vehicle, so there are some daft comments around about individual cars not being as frugal as a 525d diesel. These are ridiculous because the 450h performs more like a 535d, which produces less than 30mpg, especially on short trips. (Diesels are incredibly inefficient on short trips when the engine is cold). There are some daft comments around about fuel economy - my car does 40mpg on motorways going downhill - that kind of thing. The important thing is just to understand the average number for your kind of driving.
8) Although the 450h is extremely attractive, it's clear that the GS300 is a neglected bargain. Although hardly known, she's just as beautiful, just as reliable, and is ridiculously cheap. You get comparable real world performance for 25% less.
9) Finally, don't lose sight of the fact that new 450h's can be bought for just over £30k from Lexus, and three year old cars are retailing for less than £20k. It might be worth thinking about going for a 2007 car, or a 2008 (revised) car, as it might only be a little bit more than a 2006 price.
Bottom line - I reckon these cars are a great bargain compared to BMW & Mercedes - the challenge is finding the right deal.
Just a few, admittedly contentious, thoughts. Obviously I'm not an existing owner, but I thought you'd be interested in my perspective as a buyer like yourself.