I have used nothing but the tapco flats so far, and they work but.....
hammer holes are a bit too high. Sometimes it requires grinding/polishing the hammer to have lessened friction between it and the bolt carrier to ensure good function.
the trigger guard holes are off, which sometimes causes a pita when fitting mags. Curtis recommends enlarging the holes (slott'n to the rear) a bit if the mag fit problems occur. once riveted it will not move.
the tapco rails do not have the mag stabilizers on each side which can cause a tilted mag, and the predrilled holes in them requires slotting in order to get the rails high enough to line up with the locking lugs in the front trunion (at least in my experience).
none of these issues are show-stoppers and there have been countless builds done with the tapco flats (I am NOT attacking them).....but some may be looking forward to trying a product that is a little better, more "correct" or user friendly (I am).
Also go look at his website and look at his pre-order flat pages where you can see the differences between a regular flat, a polish underfolder, a Yugo M92, and a Bulgarian sidefolder.
A little more model specific than a tapco flat.....
Whatever works is the bottom line. I am not going to sell or replace the tapco flats that I have with others, but I am interested and looking forward to trying Curtis'.
Sort of like trying a new pocket-size cellphone after using the car "bagphone" for a decade.
FWIW
Oh yea. Also, all of the center supports (except Curtis') that I have seen (probably all from the same source) measure 1.175" in length and have no slot to help keep the bolt from hitting it. Curtis' measure 1.150" and have the slot. I may be seeing things, but the larger length center supports seem to bow out the reciever a bit. I think the 1.150" length is correct.
Again, FWIW