There's a big difference, in my opinion, between the TiNi bits and cobalt bits, especially when it comes to harder metals. A friend of mine called me a few weeks ago needing help with a bolt snapped off in his engine block, and a chromium vanadium bolt extractor snapped off rather neatly, and conveniently might I add, in the middle of it. After an hour of nothing with the TiNi coated bits, which he swears by, I got fed up with it and got my cobalt bits. After about 30 minutes, most of the extractor was eaten away with only a bit of dulling to the tip.
At $2 a bit from Northern, it's a bit pricey, but I'd be more than willing to spend it. Since I've been using them, I've noticed that the work's been going a lot faster.
That's just me though.