Despite some of the oft-heard "not as pretty as Gears" remarks, GameSpot thinks Halo 3 looks pretty good:

Halo has always had a very strong artistic vision, and the graphics have always been just good enough to convey the necessary imagery without becoming huge technical powerhouses. That's not to say that the game isn't technically impressive, because it maintains a smooth frame rate throughout, and looks very sharp overall with plenty of great lighting and other nice effects. But the visual design overpowers its technical side and really stands out. Given that the game takes place in a wider range of locales than the previous two games, you'll see a lot of different, colorful environments, including deserts, snow, jungle settings, great-looking building interiors, and more. The enemies, many of which are returning from past games, also look great.

Isn't it nice when technology serves art, and not the other way around?

GameSpot gave Halo 3 a score of 9.5 out of 10.