I remember seeing a receiver flat that had slits cut along the fold line, so it could be folded very easily, but would then have to be welded along the seam...

Anyone know who had these?

AR15plus.com

Me and a friend got some

They're more trouble than they're worth.

Are they that bad?

I have a TIG welder here at work, so I figured it would be fairly easy

Worked fine for us. Their customer service was decent too (one of them had a problem and the replaced no questions asked).

Run away.  I've done a laser-cut flat and an 80% bent blank from GunsGutsAndGod.  Go with the bent blank.  SOOOO much easier and it worked when I was done with it.  And they're about half the price.  The only good thing about the laser-cut blanks is that if you don't have a welder, it gives you a reason to buy one.  Other than something to practice your welding on, they're a gimmick.  Mine fired, but I had feeding problems and it looked pretty sad.  Bent blank.