I have a BB kydex sheath for M6. It is built very well and I am really happy with it.
Here are my observations, so take them for what it is worth.
First, I do not think you can make a comparision between no retention strap versus retention strap. No matter how good the retention of kydex is, I don't think you can make a comparison to having a strap and a snap. Now, on to the observations.
The retention of the kydex is great. It is nice and tight. Due to the fact that it sits extremely close to your body, it seems like it would be difficult to dislodge the knife. Other pluses is that you do not have to worry about water (it has a drain hole) sweat, etc. The tec-lok can also be repositioned for horizontal, cross draw, etc.
Downsides to the kydex: I do not think that I would ever put a lanyard on this knife. I would be afraid that if I got into heavy brush, the lanyard could get snagged and pull the knife out, or expose the blade. Wearing on your belt is one thing. If I clipped the tek-lok to my backpack, and took off, I am not so sure that I would feel comfortable about retention with the thing bouncing around and out of sight. For that specific scenario, I have thought about including a laynard that could loop around the tek-lok, and then snap on to my pack. The laynard would be sized such that the knife could not be pulled from the sheath. But, there are still 2 downsides to that: 1) Can't be used in an emergency. You would have to stop, open up the tek lock, unwrap the laynard, then remove the knife. 2) When you switch carry modes, the lanyard would need to be removed due to above mentioned reason. One more thing of note: I wonder if when the finish of my knife gets worn off more and more, will it effect the retension?
Neutral observations: The kydex sheath rides high on the belt. Since the tec-lok is below the handle area, it sits high on the belt. Some people like this, some don't. The longer the knife, the higher it has to be pulled to clear the sheath. It sits very tight to the body. Can be good, can be bad. I am 5'10 and 175lbs, so I have no issues with it. I can imagine if you have any extra weight hanging over your belt, the fit might be uncomfortable.
Personally, I just ordered a Mark Bartol sheath for my camp tramp. It can be seen here:
http://www.bladeforums.com/forums/showthread.php?t=449468
If I think of any more, I will let you know.