They're awful. IME, they sound good, not great, at first. Then seemingly overnight they become dull and lifeless. Soon after, they start to "fall apart." Almost like they'd been abused for months on end, but in a matter of less than 3 days. YMMV, but I was incredibly disappointed.
They have a different texture, sort of slick like Fast Fret was applied but has a gritty feeling too. I thought they sound louder, with more sustain and resonance. I keep my string action as low as possible but with minimal fret buzz, and it felt the Cobalts vibrated more and caused a little more fret buzz. They lasted a while too, still feeling fresh after weeks and a couple gigs. I have used the same gauge (Skinny Top Heavy Bottom, 10-52) of EBs for like 15 years, so I think I have a good feel for what they normally are like. Only tried Cobalts once, on one guitar. But, all my replacement packs are Cobalts so I am about to go all Cobalt for a bit. Maybe I will change all my strings today since I have some new records to listen to. I can report back in a month after use, see if my experiences are the same on multiple guitars.
Bottom line, they are a few dollars more expensive than other "normal" strings. Wouldn't hurt to try them out. I have certainly blown a lot more than $8 on trying something new.
I didn't notice any sort of slick feel like Fast Fret on them. I definitely felt the gritty feel-- kinda scratchy and harsh under the fingers. They seem to stay fresh a little longer than stock EB's, but stock EB's go dead so fast anyway, that's not really a selling point (since they're $10 a pack vs. about $4-5 for regular EB's).
I got them for cheap when best buy was clearing them out and I wish I would've bought more of them. They have a pronounced mid-range that sounds killer in a bright guitar with active humbucker. The one downside is that wound strings are somewhat coarse and I've noticed my picks wear a bit quicker.