Scott has pretty much covered it all.
I started off with an IS 200SE Auto Nav - lovely car but a bit gutless. On regular trips along the A27 between Brighton and Portsmouth, I would be regularly overtaken by anything & everything.
After a year I plumped for the IS300 Nav - never looked back. The extra power is great, the auto box is smooth as silk yet you can have great fun with the eShift. HIDs are marvellous, wouldn't go back to standard halogens. Also the car is much quieter.
Best bet is a 1yr old one (wish there had been some around when I was buying), and don't forget.... haggle!