Bad stuff:
Doing a side-by-side comparison against a 266xl and the C300, I have to say, the 266xl has a much smoother compression and is much less noticeable even when compressing as little as 3db gain reduction at 3:1. The C300 also has a faint, weird fluttering sound that I'm not very happy with. It's not noticeable through a system, but using headphones it's pretty apparent. The C300 also starts cutting high-end drastically when using anything above 3:1 with more than 6db of gain reduction.
Good stuff:
I used the C300's last night with excellent results! They really are excellent for for the money. I love their ease of use, and their versatility. I didn't want a single 4ch comp because you're obviously locked in at 4 comps, and same with a 4ch gate. With 4 C300's, I've got 8 channels of whatever I need. An ACP88 would have worked as well, and been 2 spaces less, but I've never been a huge fan, and they're up to $900. I've very happy with the purchase and feel they'll be in my B rig FOH rack for quite some time.
Oh and take the handles off the front - They look much cooler without the handles!