I have a Sensation Dressage Trail saddle that I ride in regularly and I love it! Very comfy and secure...even when my horse did a sideways spook going 20 mph down a dirt road earlier this week! I can answer most questions for you and direct you to help if I can't.
Just like with treed saddles, they aren't the best fit for everyone/every horse...but I have met a lot of people who really like the Sensations overall. I have tried several treeless and the Sensations are BY FAR my favorite!!!
As for showing in dressage, I do mostly schooling shows and NO ONE has ever questioned my saddle. I believe in the "rule book" nothing is stated about whether saddles must be treed or not. Someone else might know better, but that is what I remember out of a conversation I had with my dressage instructor. I am sure that at higher levels, it may be looked down upon by some judges (although technically if there is no rule against it, it shouldn't be held against you.....but that is human nature!).
Of course, as with most treeless saddles, you will need a good treeless pad that provides spinal clearance. In the riding photo, I am using a regular quilted pad along with a SKITO brand Interpad which provides the spinal clearance that the saddle does not provide. The non-riding photo shows an Equipedic (I believe...borrowed from a friend when I first bought the saddle). My everyday pad is now a SKITO pad shaped specifically for my saddle. It is fairly new to me, so no pics of it yet.
Yes, the seat is nice and deep...not sure if it is more so the construction of the seat or the memory foam that you sink into, but either way it is very secure. Of course, the saddle can feel different on different horses. My guy is slightly downhill (we have shims in the front of our Skitos), average withers, and a somewhat wide back (as in not A-framed), and no dip. I have no problem posting in it at all.