Lets just make things clear, the CLK-GTR is the one we want, with its cool 7 litre V12.
The CLR is a silly car that flips and we don't want it.
Theres also that really wicked CLK-LM Road Car, but I won't bring that up because it will cause more confusion.
Voted. Important informations have already been provided via links, so I'll continue with one interesting and unfortunately sad fact.
The CLK-GTR was deemed to be very unreliable. Car dealer Mark Johnston requested a full refund of his own example as it broke after travelling about 1.6 kilometres. In British car magazin Evo it appeard on the list of 10 worst cars they have tested in year 2006. They criticized usage of mechanical components inside and handling. It also suffered from a mechanical failure during the test.
But it certainly would be a great addition to other cars from this class. After all, it was featured in many racing games primarily NFS series.