I really like TT CD for Windows/Mac.
I worked in IT for 25 years. Simple suggestion that I believe would make MILLIONS OF USERS happy.
Please put the 'BACK' and 'CONTINUE' navigation buttons at the TOP of each screen.
Here's why this is important: When the buttons are at the bottom of the screen, they keep moving around depending on how long each question is. If a question is short, the buttons are higher up. If a question is long, you have to scroll down to find them.
This makes it frustrating when you're clicking through several screens, because your mouse needs to constantly move up and down to find the 'CONTINUE' button. Your eyes also have to keep searching for where the button went.
If the buttons stay in one fixed spot at the top, you can just click in the same place each time without hunting for the button. This makes reviewing multiple questions much easier and faster.