Basically they are stamped from shorter sheets of aluminum fed into standard 30 round mag dies.

They are slightly longer to accomodate the green 'anti-tilt' follower.  (They will also take a Mag-Pul follower if that turns you on.)
No one has reported any reliability issues with them... yet.  However, remember that straight 20s are generally considered the most reliable of all AR mags, especially the older ones with the alloy followers.

I bought two them from CMMG mostly out of curiosity.   I must admit that when inserted into a rifle there's something about a curved 20 that just doesn't seem right at first.   However, I quickly got used to the appearance.  As Sir Toppam Hat would say, "Usefullness before cleanliness."  (I have a 4 year old so I can get away with that...)

Here is a pic of the Bushmaster 20s, with and without ranger plates: