So far I have been using my Gould and Goodrich B890-G27 Inside Trouser Holster for pretty comfortable appendix carry. This holster works well with the Glock 26 and 19, and is also the right size for the M&P Shield.
I also got a Vedder ComfortTuck holster for the Shield that is VERY comfortable, but lacks tight retention. When the holster isn't work, there is very very little retention, but when worn it is OK.
Today, I received a Crossbreed Appendix Carry holster for my Glock 26 / 19 and I am not impressed! This is a large and wide rig. Seems to be designed for 12 o'clock carry due to how wide it is. Retention is good though. My biggest concern is how wide this is. I prefer one o'clock carry with a narrow rig, and this holster does not fit that criteria. The other issue is how high this holster rides. The back of the slide is about an inch to two inches above my belly button. So it's wide and riding high. Not the best situation for a guy with a little bit of a gut. By no means am I that big of a guy (5'7" and 170lbs.)
Comparing the Vedder ComfortTuck to the Crossbreed SuperTuck Deluxe. The Vedder feels softer against the skin out of the bag than Crossbreed, but isn't a big deal as they are both brand new. The Kydex on the Vedder is thinner than the Crossbreed (may contribute to the difference in retention). Speaking of retention, the Crossbreed blows the Vedder out of the water. Hold the Vedder upside down with the gun in it and all it takes is a LITTLE jolt and the gun falls out. The Crossbreed I haven't been able to get the gun to drop out of it with average shaking. The Crossbreed has a good amount of more noticeable retention than the Vedder, but when worn, the Vedder seems to have enough to not cause me too much concern.
Also, the combat cut of the Crossbreed fits a full high tang grip better than the Vedder. The Vedder has a small piece of felt between the metal buckle and the leather to prevent the Crossbreed squeak when moving around. The Vedder seems to be a better put together holster than the Crossbreed and if it wasn't for the decreased retention, it would blow the Crossbreed holster out of the water!
I'm still trying to give it some time with the appendix carry holsters from Crossbreed to see if I will be keeping them. May try a Raven Concealment appendix carry rig to see if that is better than the Crossbreed.