I spent one evening with Flinto, from start to finish, and I went from zero to complete prototype that wowed my clients!
I have tried MANY many many many... prototyping workflows and products and services. Ideally, you need to be flexible to have a couple under your belt because some work better on a device versus desktop. I used the Flinto app to import my Sketch design and animated the transitions from slide to slide so easy, I kept telling myself that this is too good to be true. But it wasn't!
If you need to show a prototype on a device such as an iPad or iPhone, this is definitely something to consider!
For rapid prototyping, this is probably the best option for you just based on the speed with which you can accomplish what you want and get client feedback.
It crashes more than you want it to, but less than others.
I wish that there was some more fine grained control over animation, but that is more of a fringe case. The more you use it the more you wish it could do, but honestly I believe the app is very good at what it does without trying to do too much.
I wish we could integrate with some more advanced functionality to add Json data to our apps to at least display sample data.