Great IMHO. Racers overall seem to have struggled this gen - the marketplace has been split across multiple consoles like never before, and genres such as FPS are outselling everything in sight.
Therefore, over 2M at launch vs around 2.5M for GT4, after wacky delays, way too much hype and a few mixed (well, lower then expeced at 7) reviews were bound to make a dent in it. Plus in US Kinect is still in its first flush and Wii was getting it's seasonal boost.
So I think 2m is pretty damn good - certainly more than most racers on multiple consoles.
Now though, it's all about the legs. If those are weaker then I'd feel it underperformed a bit - if they look good then I think the overall LTD will be favourable.