I get a lot of questions about 20mm through axle forks (rigid ones, that, is). Here are some thoughts and a few photos of Jefe's fork before it went off to powdercoat.
-I really think through axles are mandatory for any fork over about 100mm of travel, especially for big folks and especially on 29ers. The difference is really night and day. That's suspension forks, of course. For rigid forks, most people won't notice a huge difference in the ride - rigid setups are already really laterally stiff, even with a standard 9mm QR axle. For big or aggro riders, or folks who really want the ultimate in steering precision, though, there's no argument from me that this is the stiffest setup you can get, short of doing a rigid fork with a TA AND a 1.5" steerer.
-It's a lot of work to build the dropouts and attach them. Really small stuff (ie, pinch bolt dropouts for 20mm axles) is always a pain. It probably takes me an extra 2-3 hours of work to do a TA fork versus a standard dropout model. Consequently, they cost an extra $100. Not cheap. Probably only worth it if you're a serious snob or have a *really* fancy TA front wheel you want to swap back and forth from a suspension fork.
-The actual build process is the same as for a standard fork, but I cut off the bottoms of the fork blades at a 35 degree angle, cut a piece of .065" 4130 sheet to fit, fusion weld, and file smooth, then make a 7/8" miter for the actual dropout, which is made from (you guessed it) 7/8" 4130 tubing and a seat binder. I like to use the seat binders because the M5 bolts are a lot harder to strip or screw up than the M4s you find on a lot of stem clamps and such. The dropout has to be welded in *really* carefully (and mitered perfectly) to keep the fork in alignment and make sure the axle will fit smoothly and securely when installing the wheel. It's much, much touchier than doing a standard DO fork.
-Weight is about identical to using standard dropouts. There's probably a 10g or so difference one way or the other, depending on what 9mm configuration you compare to, but for all practical purposes, the weight is the same.
So anyway, I build a decent number of these, and I'm happy to do them in any configuration you can dream up, as always.