I played Elixirs since 2007,and tried a set of cobalts on a Bongo 5. Bought 3 more 5 string sets,and two 4 string sets. They sound that good. The biggest thing I noticed was the clarity of the B string. Very focused without a lot of excessive overtones.
http://www.talkbass.com/forum/f16/elixir-steels-vs-eb-cobalts-my-little-review-865137/ http://www.talkbass.com/forum/f16/new-eb-cobalt-srings-859835/ http://www.talkbass.com/forum/f16/new-eb-cobalt-strings-846034/
I've got them on my bass right now. I think they have more punch and maybe a little more fullness on the lighter strings than the nickel EBs. I don't think they are brighter than the nickel Slinkys though. The brightness is about the same to me. They are nice strings.
Nickel or Steel? I compared them the nickel versions of both...Cobalts are way brighter, punchier, and louder than both. Didn't compare them to steels.