I use Ninja Forms for my work - it is our primary method of user submission. I was able to pick up its interface very quickly and get right to creating the forms needed in short time. The support is fantastic - every time I have emailed with a question, I've received a response within half an hour - usually less.
I have used the Conditional and Layout/Styles additions, and both work well for what they are intended to do.
The only problem I have encountered so far is with Conditionals sometimes not saving properly and acting screwy - eventually I am always able to get them working, but it adds a bit of extra time to make sure they all work properly.
I will continue to use Ninja Forms and I recommend it highly.