I feed collards in a limited amount, only due to offering a variety.
Collards are my favorite green to get if I am going away for a weekend. I can drop in a head of collard greens, and they hold up decently well for 24-48 hours, where as normal greens and spring mix will rot or dry out well before that time.

I have not read any bad reviews on collards, however, moderation is key for all offerings...