I've actually had the FXC80's in the past also. The fit is better than most say it is, and I find them very comfortable either worn up or down. The problem is the sound. The treble is incredibly hot and fatiguing and EQ doesn't remedy that. These DO NOT respond well to any sort of EQ'ing, as doing so seriously distorts and ruins the sound. I got rid of them quite fast. Actually, I just snipped the cords off and use them as $30 earplugs now...
The FXC51 is definitely a step up from the marvelous FX67. But you MUST EQ them. FX67 wasn't required that you did so, but these, you need to. Not saying it sounds bad without one, but it's nowhere near as close as if you apply one. Cowon owners need to pick a pair of these up! I'll share with you my favorite EQ I've been using, and you can try it out for yourself to see if you like it. But anyway, incredibly impressive headphones in general.
Regarding the fit, they isolate MUCH better than the FX67, yet they are nearly on par with comfort. Impressive. I would say that have an even better fit than the FX67, making them not only the best fitting / most comfortable IEM I've worn, but also (mentioned before) the best sounding I've heard. Due to all this, I have never had the slightest itch to upgrade or try something different. My portable rig is set in stone, for good. I wish the same could be said for my desktop rig...