To quote the review on this site:
'Graphically, Mass Effect 2 is an improvement on the first game, but not a major one, and while the game looks good it does not compete with the top contenders in visual fidelity on either PC or Xbox 360.'
Given I quoted it, it probably comes as no surprise that I completely agree with it. I've completed the game twice now, and while I disagree with some of the conclusions of the review, the above is pretty much exactly spot on.