Dang, sorry I missed this for a year!
I have several "traditional" drillings and a D99 Duo, so maybe I can be of some help here.
I consider the D99 to be a superior design evolution, because it solves a couple of problems.
The way the rifle barrel is floated on the D99 greatly reduces the uneven transfer of heat into the barrels which causes a POI shift in the traditional drilling design on longer shot strings. Not a problem for a hunting gun really but quite annoying at the range if you like to try new loads for best group for example.
All of my drillings seem to be uncommonly accurate. I guess this is due to the stiffness in the traditional design. The D99 has a pencil thin barrel that is attached to the others at the muzzle. I'm not sure what kind of voodoo Blaser puts in it but accuracy is outstanding. The D99 shoots so well it has encouraged me to get another barrel set for it. I've been waiting 15 months as of right now so I hope it comes eventually! My one gripe with Blaser is they use marginal twist rates in almost everything. The 6.5x55 barrel is only 1 in 8.66" and won't stabilize really long bullets like the 143 ELD-X, but 140 grain spire points work fine.
Love the saddle mount for the scope on the D99. Just so much easier than the hassle of replacing a claw mounted scope. Simple to have a second mount with a thermal scope for night hunting as well. Just makes the gun much more versatile.
I was somewhat worried about the rear sight leaf not folding down for shotgunning like a normal drilling. I took it to my sporting clays course and shot with it and it worked fine though, I didn't notice it really.
It is too bad Blaser didn't include screw-in chokes for the shotgun, but this can probably be added.
The "Duo" isn't offered anymore, but the second rifle barrel works great and is easily regulated. My combination is 6.5x55, 22 hornet, 20ga. I have a 9.3x74r/20ga combo on order.
Just killed the biggest deer of my life with the D99 last month. I have the plainest version and use it quite a lot, it's probably my favorite gun. The metal finish is corrosion resistant. While I try and take care of it I'm not afraid to drag it through the woods or hunt in the rain.