I'm not convinced there have been widespread transmission problems on the RX300. I always check Consumer Reports before I buy a car and it's almost never steered my wrong. Their used-car ratings (which are compiled directly from feedback from owners) rate the RX300 transmission as follows:
1999 - good
2000 & 1 - very good
2002 & 3 - excellent
2004 - good
2005 & 6 - excellent
Furthermore, there are amazingly few areas that are rated just "good" or less. They are:
1999, Exhaust - fair
2004 - Brakes, Body Integrity, Power Equipment - good
2005 - Body Integrity - good
Everything else, on every single year is rated either "very good" or "excellent."
Now, one thing you definitely need to bear in mind regarding newer used models is that it may take awhile for the problems to surface. So, while CR may rate everything good, very good, or excellent on a certain late-model used car, that may well change as the years pass and things start breaking down. Which is why I said CR has "almost" never steered me wrong. My last car was a '99 Subaru Outback and it had stellar marks on CR when I bought it in 2000. But as the cars got a year or two or three older, owners started reporting problems and when you look at their ratings for the 99 Outback today that is clearly evidenced by the revised "poor" rating it gets for major engine problems, among other things. (Which is why I finally traded the sucker in on an RX300 yesterday!) But, while I don't consider a 2003 to be a "newer used model," I also know from experience with my Outback that it may take more than a couple of years for problems to surface. I didn't start having serious problems with my Outback until 2002-3 when it was almost five years old.
So I was at first a bit concerned when I got online last night after driving my "new" 2003 RX300 home and saw numerous complaints about the transmission on various model years. But if it really were a widespread problem I have to believe that would be reflected in the CR ratings by now, and it clearly is not.
Another thing to keep in mind is that unhappy people are much more likely to gripe and moan and complain on internet boards than happy ones. CR's owner surveys are, by their very nature, going to gather a much greater cross-section and will, therefore, give you a much more balanced assessment.
All of that said, I gotta admit I'm glad I popped for a 4-year/48,000 mile extended warranty that covers virtually everything so I don't have a lot to lose if the car does turn sour on me. ;-)
Mags