steve - i used cowboy brand and also kingsford lump last year with my ECB - everything seemed fine - temps might have been a little higher but no other real issues. it seems that good charcoal briquettes (kingsford or western family are the ones i've used with best results, but royal oak would also fit in here) you are able to maintain a consistent temperature, whereas with lump it seems you have to babysit it a bit. no big deal, as it builds character, right?
one positive is that being from hardwood, you certainly get some sweet-smelling burning, and i do believe that improves the flavor of the meat.
those are my experiences. i got some green bag royal oak (they finally have it up here) and am looking forward to trying it.