It really depends on whether or not you think you will actually USE the navigation function. I have a Toyota Highlander with nav (essentially the same system as the RX - as you know Toyota and Lexus are one in the same) I waited an extra month to get the one WITH the navigation becuase I wanted it so badly - now I would do anything to go back and get one WITHOUT it. Everyone has a different opinion, but this is mine:
The map and all the other features were fun to look at for about 3 weeks. I think I have actually needed to use the map only once. As it stands now, the things that irritate me about having it are that you have to hit the "I Agree" button EVERYTIME you start the car before you can use the mapping and anytime you push a button on OR off the screen (temp control, settings, etc), it makes an annoying beep (this may or may not be the same in an RX, but I recall trying one at the dealer and it was the same)
The back up camera is a different story. This may actually be something that you could use and may help you avoid an accident at some point.
If your reason for getting the navigation was strictly because you think it will be "neat" to have, then don't do it. If you actually think you have a need for it to find the locations of things or if you take trips alot, then the $2K is worth it.
If you have any other questions, you can e-mail me at JohnJCronin@aol.com