1. Starter failure at 101k kms. A known failure with high mileage. It's expensive to replace because it sits in the valley of the engine v-block underneath the intake manifold, so there is plenty of labor involved. I was told by a mechanic that because of the location of the starter, it's subjected to a lot of heat, which causes it to eventually die.
2. Worn front suspension bushings. Again, a known problem with high mileage. Lexus made them soft for a good ride, but as a result, they don't last, especially when taking into consideration how heavy this car is. Common symptoms are wandering in the steering, kickback under hard braking, etc. It's an expensive replacement, as you cannot buy the bushings alone... you have to buy new suspension components with the bushings already installed. There is a solution... Daizen makes aftermarket bushings for the GS... but finding a mechanic willing to deal with suspension parts that were not designed to have the OEM bushings come out is a problem I haven't figured out.
3. Headlight condensation. The seals are known to fail. If the car is still under warranty, it would have been replaced no charge. But I'm not under warranty >_<
4. Rear main engine seal. It leaked right after I bought it, and did a major scheduled service. I was in tears, because I had done a mechanical inspection on it prior to buying, and it was not leaking then (I did not buy from a Lexus dealer). But a product called Auto-RX saved me and it has not leaked since (http://www.auto-RX.com).
5. Sunroof shade rattles. There is a fix for it involving some kind of spacer, but I haven't been able to make it work =(
6. Bad ignition switch. Previous owner must have had a 50lb key chain, because it had damaged the tumblers inside the ignition, which sometimes means I cannot get the car to start. It's expensive to repair, because it involves pulling out the entire steering column, so right now I'm following the advice of the mechanic, and making sure the key is absolutely perpendicular and fully pressed into the ignition to ensure startup.
7. I have an intermittent coolant smell inside the car. I get this maybe 1-2 times every few months, and anytime outside of these times it's perfectly fine. It was originally diagnosed as a leaking heater core, but the mechanic is doubtful that is the cause, because it's not constant and is still present even when the HVAC is turned off. There are no visible exterior coolant leaks, no loss of coolant in the radiator, or anything in the oil. This one still remains a mystery.
The car is highly reliable for it's age and mileage. When I was shopping around I was also considering a BMW 540i because I loved how it performed and handled, which the GS cannot compete against. But when I read the horror stories about the 5-series reliability, I know I made the right choice.
I'm 6'2" and I find it roomy enough. We used to have a 1992 Jaguar XJ40, and the increase in interior volume is immediately apparent, but the Jaguar's wood and leather were much nicer than in the Lexus.
All the interior controls made sense to me the minute I sat in the car for the first time... a very good sign to me, especially if you have struggled through something like BMW's I-Drive with manual in hand just to turn on the damn radio >_<. However, the location of the mirror, fuel door, and trunk release controls could be better, but it's better than where they put it in the 2006 model.
The suspension is soft. Fortunately my GS came with TEIN coilovers, which made a huge difference.
The transmission is a 5-speed auto, and it's very smooth most of the time. When it's cold it's slow to shift, and sometimes can be tripped up when caught between gears at slow traffic speeds. I do like the manual e-shift buttons on the steering wheel... but from what I have read online, I am one of the few who actually use it.
I was worried about trunk space at first, but so far I haven't had a moment in which I couldn't fit something into it (within reason). I wish there was some sort of pass-thru between the rear seats though.
Brakes could be better. The car is heavy and the brakes only seem to feel adequate. I know the GS recorded short braking distances in magazine reviews... but I always feel it could be better. Supra twin turbo front brakes are a direct bolt-on... but I would need to find new wheels because my GS wheels will not clear.
Steering is not as nice as a BMW, but it gets the job done. Turning circle is surprisingly tight for a car this size.