I just got my Vortex keycaps a few days ago and I must say that I really like their look!
Unfortunately my keyboard hasn't arrived yet, so I can't say anything about the feeling when you type.
The only bad thing I noticed to this point is a little bump on the rear side of each key:
Show Image
(http://i.imgur.com/TSl1vFG.jpg)
But since it is on the back, you probably won't notice it.
But a blank set is also pretty cool! ;)
They're called "spruce marks", from the mold, and Vortex seems to be infamous for them. I have them on a couple of mini sets of PBTs of theirs (a Poker X and a Pure). They're not a sign of anything wrong with the keycap, and, like you say, they're on the rear of the keycap and can't be seen, but Vortex really ought to clean up their act and/or molds.