I recently created a thread in this section of the forum asking about the reliability of the CVT transmissions, and all of the responses were positive. I've researched the "design" of the transmission, and it doesn't seem like there's a lot of stress on it. I also haven't heard of a single complaint about the transmission on the Rx400h, which is a good sign.
I love my Rx400h. I got mine used about a year ago (in 9 days it'll be a year) from a little dealer near by. I haven't had any "problems" with it, just a few issues getting some TSB work done...
Have you looked at any Rx400h's in person yet? There were a few changes made for the 2007 year, many upgrades including the navigation screen resolution, and interior wood trim replacing the 06's brushed aluminum trim. In 2008 even more changes were made, including blue tinted headlights and taillights, and blue accented emblems- just cosmetic things.
Most (if not all) Rx400h's (and Rx350's) have a known defect of blowing out a white powder through the AC vents, due to a poorly designed evaporator core that flakes as it ages. There is a TSB (or recall) for the fix, and it's covered for free. I had the fix completed this past March, but when i got the car back from the dealer they had scratched my radio, nav screen, nav unit and some vinyl and wood pieces... It was a hassle but i got them to replace the parts.
I think the Rx400h is a very reliable and wonderful car (well, SUV). It's built VERY well and the fit and finish is pretty amazing. It's a little more powerful than the standard Rx350, actually the Rx400h is the fastest SUV Lexus has ever made, even faster than the 010 Rx350 or Rx450h! Just don't buy the Rx400h if you think you're going to save on gas, because the gain is pretty slim. I think the Rx400h is more of a performance hybrid... well sort of. It seems like Lexus had a hard time figuring that one out.