We traded our '99 RX300 with 135,000 miles on it. No transmission issues, but we did have the "engine sludge" problem.
The Lexus did seem to have some issues past 90,000 miles, but overall it was a very nice vehicle.
I have a 2000 RX300 with 225,000 miles without many problems, but had to replace A/C servo motors, $600, and plug wiring, which was $800; in about 40k miles that is not bad.
I own a 95 Lexus LS400. My grandfather bought it for me as a graduation present in 97 and it had roughly 40k miles. It currently has around 225k and still moves like a rocket down the road. It was fully serviced at 150k and the starter went out at around 200k. Overall it has been a WONDERFUL and DEPENDABLE vehicle.
I have a 1999 RX300 with 159K miles. I bought it used at about 37K miles. I haven't had any transmission problems, but was made aware of them early on, and have had the transmission serviced every 30K miles or so.
No engine sludge problems - have had frequent oil changes also. I did have to replace the idle air control valve, which is one of the common issues, and I just replaced a rear wheel bearing which was apparently a very rare occurrence (and a $650 bill).
However, I have bad the "Check Engine" light problem off and on since about 120K miles. I have had one O2 sensor replaced for about $600, but the light didn't go off. It comes and goes, and is staying more frequently now. It is always the "lean bank 1" error.
I'll drive it until it dies.
I just sold my RX 300 with 289,000 miles and purchased another 1999 RX 300 that looks identical to the first one. This has been a great vehicle with only a few minor problems. I have never replaced the transmission or anything major, so we must have gotten a gem. Hope this one is as good.
I own a 2002 Lexus RX300. I believe that if you take proper care of your car it will never let you down. I love my car and I consider it very dependable. I have never had the "check engine" light come on, and have over 100,000 miles on it. Lexus parts are not that expensive compared to other luxury cars. So, if someone is trying to charge an exaggerate sum of money to fix any problem, I recommend you find another mechanic. Keep up with the regular maintenance and this car will never let you down!