I have a NiteRider Sentry Aero 260 which is a cob light that supposedly puts out 260 lumens; I also have a NR Omega 300 that is not a cob light and puts out about 300 lumens. Note that both of them put out nearly the same lumens.
So here's what found out, there is a major flaw in cob lights, during the day in direct sunlight the sun completely washes out the cob so it virtually disappears while the other one is startling bright. At night the cob does just fine, but it isn't near as bright as the other. So I would say that cob lights put out about 1/2 the lumens they claim they put out.
I bought the Sentry Aero 260 because of it's very good side lighting, so that one gets attached to my helmet when I ride at night, while the Omega 300 stays attached to my saddle bag day and night.
My experience with other people using cob lights have been similar, so I don't think it's just my two lights. When watch other cyclists from behind the standard style LED is noticeably brighter than cob style.