I took the CTD two weeks ago through PSI. The PSI experience is passable, but annoying. The first available test times are a couple weeks out, and then those slots are all outside normal working hours. Think exam slots starting at 0-dark-thirty, or after everyone else has gone to bed. They really want you to have nothing in eyesight when you take the test. No post-it notes, no paper, no extra keyboard, no photos of the wife, no food or water.
The exam I took was I think quite reasonable. The exam tested similar skills as the practice exam. Time pressure was about equal to the practice exam, and that was not bad at all. I've been actively using teststand in my day to day job for 2 years, and I think the CTD exam is quite good from a practical perspective. It wasn't harder than what I'd expect a developer to be able to do in the time allowed. It also didn't instill negative skills like the CLD did when I took it 2 years ago. That exam soured me on the whole NI certification thing. Maybe I'll try the CLA after all based on the CTD applicability.
Now if my CTD exam comes back with a failing grade, then I'd probably not retake it.
For anyone looking to prepare for the CTD, study the prep materials and then repeat the practice exam until you can complete that in under 2.5 hours, then schedule the exam.
Expect no confirmation after completing the exam, and expect to wait a while for results.