I got Dr. Cooks bitless bridle for christmas, to help train my 4yo paint, she does fantastic in it. I really like it, it took her a little bit to get used to it, for the simple reason I rode her in a halter for all that time. It takes some getting used to, but its great for green horses just learning the ropes. :)
I don't have this bridle, but I do have the Nurtural, both the pretty leather one and the synthetic one, and I have read some reviews on the biothane bridle. What I have heard, is that it slips very easily (the Nurtural won't because of the rubber inlay in the noseband) and the release is insufficient (same goes for the synthetic Nurtural, the material just isn't perfect). I also got a remake of the Dr. Cook bridle, with round-sewed cheek straps. My horse goes great in both that one and the leather Nurtural, only the communication is just a bit more precise in the Nurtural because the cheek straps won't interfere with each other. I'd swear by this bridle any time. You can ride in it just as you would with a bit and the horse gets the hang really soon. I bought it, lunged my horse in it with running sidereins a few times, got up on him, rode away and never used a bit again.
With this bridle I've also finally managed to ride my horse alone through the neighbourhood (he reacts violently to a bit, will throw up his head and bolt back to the barn), and even the one time he got seriously scared and bolted, I let him go for about 20 meters as to not pull on him right away and give him reason to fight me, then gently pulled him back up with no problem whatsoever.
IMO the Nurtural (and any cross-over cheek strap bridle) is great for more specific riding like dressage because the cueing is a bit more advanced. For just some strolling around or trail riding, I think a sidepull or rope halter would work just as well (I tried them all on my horse but found the possible communication to be lacking for dressage).
The more I research bitless bridles, they seem to be varioations of the war bridle. They all work on a scissoring effect so the release is most important. Unlike a snaffle where whereby the rider can elicit a response by merely stroking the rein with the pinkie, creating lightness, this cannot be done in a bitless.