I loved my C5, and hated to even think of selling it. After driving the C6 home from the museum, it wasn't very hard to sell the C5.
Just from a performance view, the '05-'07 Z51 C6 coupe is essentially identical to a Z06 C5, and now even better for '08.
Many who don't like the C6, IMO, don't like it because they say it's just a C5 1/2. They say there weren't any major improvements. As someone who's owned both, I can tell you that they just might be right. However, there are very few things in the C6 that don't have at least a little bit of improvement over the C5. Add all of those little things up, and the C6 is one kick azz car that will not be mistaken for a C5 by anyone who has driven both.
As far as looks, they are pretty close, but I've had more comments from complete strangers about my LMB C6 in the past 9 months than I had in 3 years of owning my Silver '01. Sure, the headlights don't pop up any more, but the C6 HID's are 1000% better at lighting up the road than the crappy OEM C5 headlights were.
Best advice I can give is this, buy what you WANT. Do all of the research that you need to, decide what options you must have, what color combos are acceptable, then don't settle for a car that doesn't meet your minimum requirements. When shopping for a C6 I couldn't find what I wanted on a lot that was east of the Mississippi River, so I ordered one. The wait was well worth it.
C5's are GREAT cars, and the C6 is just the slightly improved next generation. You'll be happy with either one as long as you do your research and get the car that you want.
