Forget the E6600, my favorite will always be the E6300. Not only was Conroe itself a crazy strong performance bump at the time, but $183 for a dual-core chip was amazing. The E6300 was a paltry 1.86Ghz but a very mild voltage bump would let you run it at a full 100% overclock 24/7 stable for nearly equal performance as an overclocked $999 X6800. The CPU wasn't even the bottleneck, it was the FSB capability of the motherboard. I ran my E6300 at 3.8Ghz for years on a P35 chipset board, 24/7 stable and it didn't even get particularly warm doing it. Sure the Q6600 eventually got cheap a few years later, but not even good watercooling could tame that chip's heat output at 4Ghz.
Almost got 3.9Ghz stable out of that E6300 too, but there just wasn't enough FSB headroom left on the motherboard to make it reliable. Maybe some super expensive P35 board might've done it, but at that point it would no longer have been such a crazy good value chip which would defeat the purpose.