I have an 02 LS430.  The only problem I've had with it was the side
mirrors that acted up and both had to be replaced (under warranty, at
$700 each, I'm glad it WAS under warranty).

Besides that mishap, I've had zero problems with the car.  I love it.

Assuming your purchasing from Lexus - you said 'certified' - nothing to look
for other than price.
As bullet proof as a car can be, see the number of dissatisfied S class
owners.
Maybe a bit bland for some, cabin particularly.
Servicing costs are reasonable and garages seem genuinely helpful.

I own a 2002 LS430, silver, modern luxury package with the navigation.
It all depends on what you are looking for in a car.  What made you
think of the LS?  I sold my 2005 330i BMW to get this car.  Yes, they
are totally different cars.  I like both.  But after owning the LS430
for a year, there is little to complain about.  Nearly all options are
standard.  No such thing as a base model LS430.  I haven't run into any
LS owners that had anything bad to say.  If you want a smooth, ultra
quite, comfortable seats, and good power, than the LS430 is a car you
will not regrete.  I've own many cars in the past five years (BMW's
3series 5series and 7series, Lexus LS400, LS430, GS300, IS300, IS350)
and I like them all and each for different reasons.  I have lots of
friends with Accords and Acuras.  They are very simular to Lexus
owners.  They like the reliable, safe, and solid cars both companies
build.  If you had the high end V6 Accord, I would wonder why you would
part with it.  I'm not saying both cars are the same, but I think both
are good car for the price range.  The bottom line:  I would take the
LS because you will not be looking for another car for awhile.

I took a 01 for a ride (looking at 01 and 02's) today, I have been a LS400
(1996) owner for about 5 years now and very, very happy with it. I drove the
01 today (with nav and all the toys), and was blow away. The LS400 was great
but, the 430 is a very nice step up.

I don't own a 430 yet...but, I will, very soon, it's that great of a car...

BTW: I don't think I would ever look at another car after driving my LS for
about 5 years, so, take my thoughts with a grain of salt...