But the biggest sin of all, and even Top Gear mentioned this, is the lack of leg room in the backseat. Even my Xantia has better leg room there than the C6. And maintenance wise, it's a nightmare. It's nowhere as rock-solid as the XM or as "simple" as a DS. While it's probably a good investment, considering its few numbers (only 23k were made, I believe), and the fact that it's interesting, it's probably best to mostly keep it in a garage for one or two decades.
I really wanted to be excited about the C6, but it's so much further down my Citroën shopping list than even a Dyane 6 (and I'm mostly into Citroën for the hydropneumatic suspension).