I think they're great for rock. For me, I think they're a perfect match for my favorite band Pearl Jam. I don't listen to much rock, but I think they'll be very good for that. I won't lie, the DJ100 is only going to be only good or close to very good for metal. For metal, most of the Grado headphones are a better match. I think the DJ100 actually an all around good headphone for most any genre. Very few headphones are great for all of them. DJ100's weakest genres are maybe metal and classical. I actually think classical is just fine on the DJ100, but some may not agree. For classical I'd almost always prefer an open headphone.
BTW I can't think of anything the UR-55 does better than the DJ100. I own two pairs of them! I think when modded the UR-55 does have a better soundstage, but when modded it's kind of open. It also may have something to do with the foam that's used.
The UR-55 has even more rolled off highs than the DJ100! It's sound signature is more like the Porta Pro.