I use the Milwaukee ones in various lengths pretty much daily, and they work fine. Only thing is the head is a little big, so they won't chase through the whole if it's not at least 1-1/8".
I have had good luck with bosch extensions in my impacts. Im unsure of the brand but I have an older 16" hex extension with 2 hex set screws I sometimes use in my hole hog and It been a helpful tool.
I have a 6in craftsman magnestic thats served me well for probably 15 years. And have an old black&decker 12in extension with two set screws thats also great. Milwaukee stuff is generally good so i dont see a problem with it.
I've got the Milwaukee 7/16" quik-lok extensions in numerous sizes....they are VERY tight fitting & have almost no play/slop at all. I have a complete review coming out for the new M18 FUEL Holehawg w/quick-change chuck (the same one that is on these extensions). I will be sure to use the various sizes of extensions in that video so you can see for yourself.
The comment that the prior reply had about the size of them is accurate. The quick-change end is larger than one that you would simply tighten down with an allen key, but that would be the only downfall as far as I can tell.