I've tried it, but haven't been able to make it work yet. I'm 5' 9", 180 lbs, and a good 20 lbs overweight, so I've got a gut. I have two problems with appendix carry which I have been unable to solve: 1) my gut pushes the butt of the gun outwards, resulting in a lot of printing and 2) when sitting down the muzzle jabs into me, and becomes painful relatively quickly.
I've tried appendix carry with a Glock 19/23, and I found that the butt just printed too much. None of the appendix holsters that I've tried pulled the butt in tight enough. I ended up switching to a Glock 26, which helped the printing somewhat. The shorter butt of the Glock 26 made it easier to conceal, but it was still thick, and my gut still made it print.
I tried the Raven Concealment VG2. It is a minimalist holsters, with very little bulk. It was the most comfortable. Unfortunately, it let the gun move around too much. The gun would rotate butt downwards, and the butt would print like crap.
I tried the Comp Tac 2 o'clock holster. I found it terribly uncomfortable and it didn't hold the gun in tight enough.
I tried the Edgeworks Incog holster. It was more comfortable than the Comp Tac, less comfortable than the VG2, and still printed too much.
Perhaps my next try with appendix carry should be with one of my Kahrs. They are a lot thinner than the Glocks. I've got no idea how some of you can appendix carry a gun as large as a Glock 17. If I sat down, that slide would be jammed into my pelvis, the butt would be levered out by my gut, and I would be one unhappy camper.
Raven Concealment has come out with a new appendix carry holster, the Eidolon. It looks to solve the problem of the butt sticking out, but it looks bulky as hell.