Just like my poll about Sonic Adventure, I'm playing through Luigi's Mansion right now and am hoping to beat it within the next couple days. It has an interesting place in history, as the GameCube game launched without a true Mario game but instead with Luigi's Mansion, which has almost nothing in common with any of the platforming Mario games.
So... Was Luigi's Mansion a good replacement? Cheap, quick game to try to move consoles? Great expansion of the Mario franchise? And... do you want another?