I'm using GW ones (and they're really good). Also I've made some experiences with Vallejo black one. It's also not bad. But GW is better.
I would recommend GW Chaos Black as well. It sells in Germany for about 11,50 EUR I think.
GW doesn't sell primer. They sell black paint. GW stopped selling their Chaos Black as a primer years ago. This is a very important distinction. GW might tell you to use it as a primer but I'd hesitate to use it.
I think its very inferior to actual primers and if you paint miniatures with a significant amount of detail in them then you really notice the extent to which the GW Chaos Black fills in details.
Check your local hardware store and see if you can find black automotive primer. Its very fine (as in the texture and density of the paint), covers well, and, most importantly, doesn't fill in details on your figs.