my g&g highcaps for my m14 feed everything but 5 or 6 rounds... what sucks is counting them as they go in if you want to be precise.
The other thing that sucks is even half or quarter loaded (or even realcap + compensation to account for the ones that get stuck in the feed tube or the floaters that don't get pushed into the hopup), is that they are loud... they rattle... and you can hear a guy with a highcap trying to be quiet. Though it's not as bad as people make it out to be...
I usually hear them stepping on twigs/sticks or talking with their buddy long before I hear the rattle of the highcap.
BTW, if you're handy there's ways to quiet them down with materials like foam or dynamat on the inside. It takes away capacity, but can cut the audible noise down quite a bit.