I haven't had them for very long, maybe 2-3 months. I bought various sizes from the same brand to replace some that had worn out (black, not cobalt). They drill very well for now, clearly faster than the all-black ones when they were new. If they also prove to be durable, it will be a successful purchase because their price is very good.

update 14/11/21:
It has been over a year since I started using them and I can say that they are worth it! I have all sizes up to 12.5 and I can confidently say that they drill better than the plain black ones, by far! In the smaller sizes, which usually broke easily with the black ones, now I see that they don't break as easily. The cobalt alloy is a serious investment. Now I have an opinion, I like the drills from IZAR as they make your work better and you don't pay a fortune for them (even if they turn gold :-).