Cobalt is nice, as it allows running at a greater sfpm than HSS. The big drawback to cobalt is that almost all M42 drills have unusually thick webs, which isn't a problem until they get dull & need to be sharpened. Very few drill grinders will properly thin the web - neither my Darex V290 nor the Lisle 9100 can handle a cobalt bit. This means sending the drill out to a commercial facility, or learning to thin the web by hand.
I got on a cobalt kick a few years ago & bought some for a drill & tap job. The taps were NC 1/2", NC 5/8", and NC 3/4", nothing really big or really tiny. The cobalt drills did a great job until they dulled, and I discovered the difficulty of getting them sharp again.