The learning curve is relatively low compared to other software that supports animations/transitions. Both building the wireframes and designing the interactions is all drop and drag.
You can simulate all the typical iOS gestures such as tap, double tap, swipe & long hold. These gestures then can trigger animated transitions such as sliding, fading, pop in, & flip. This enables you to create very realistic interactable demos of your app. Checkout the demo page for some examples.
The UI library is huge and very high quality. All the UI required to design iO7 apps is included as well as iOS 6 and basic wireframing elements. For example in iOS7 it provides:
Backgrounds
Navigation & status bars
Layouts (list pickers, maps, keyboards, scroll etc)
Controls
Typography
Icons
Fluid isn't just about wireframing static screens. You can plan out all the user flows and then define the transitions between them. This is really handy for planning out the entire user workflow.
We were a paying user of FluidUI at the beginning of October 2014, and it seems like weekly there are server outages, our work has been lost, features have been removed (zooming for one). Other times we cannot select controls, cannot edit text, cannot resize items, etc.
Often I simply receive a notice stating 'Too many connections' when trying to view my prototype.
Contacting support is next to useless as bugs are rarely addressed, but new 'features' make their way in the app, such as repositioning the screen on whatever control I click on so my screen is moving around more than the Teacup ride at Disneyland.
Pidoco's screen map view lets you easily link individual prototype pages. This is also great for uploading screenshots and linking them via action hotspots.
UXPin offers elements from a wide range libraries. This includes responsive frameworks such as Bootstrap and Foundation, and libraries specific to mobile prototyping.
$29 per user/mo ($350 pa) will soon rack up compared to a standalone product like Axure PRO ($450) which has a license that lasts years (they have given out free updates for the last 2 versions).
It's a one-page app based on HTML5, you won't see loading bar once the app is loaded, the work flow is very smooth and fast, you'll feel it's like a native app.
You can export HTML bundles to desktop, push to PHP/Relational Work environment, push to REST based API/Marketing dev space with built in Adobe Analytics.
Use Illustrator to start UX. Exports to JSX/SVG universally used in ANY Render/Runtime. View in XML (symbols) is native. Layers exports as DIVs. Groups export as g's (D3). Mapping GIS/CAD based 3d-WebGL can be fully functional from usable code generated in comp.
You will have to fully understand Javascripting/XML to understand Comp to Code conversions. Minifications and "nodes" can occur with base 64 and set up within application. Development and understanding of components would have to be set up and understood prior to "design" phase.