Since the IS 250 is a new design for motor and body, and AWD is also new to the line, there's no way to really know. Based on my experience with a 2002 IS300, 37,000 miles and owned it 2.5 years, really reliable. I've had zero problems with mine. The first model year there were some problems, but nothing serious enough to regret, from what I hear. Of course the IS300 is based on the Japanese Altezza, which already had a couple of years before they started making the US version.
Lexus, overall has a much better reliability than Mercedes over the last 3 or 4 years for sure.