Axure is extremely easy to learn on the job. Their forums are extremely helpful, and a quick search on Google yields all sorts of free widgets, tutorials and prototypes for download. I found the ramp up time is very low if learning to use Axure. Additionally for light prototypes used for testing, Axure is a wonderful tool.

I like that you can create almost any interaction needed. Dynamic panels are very helpful.

All of the advanced features that other prototyping software doesn't have. I am used to create high fidelity prototypes fast. Combining ease of use with high functionality is the best feature of Axure. Time to try version 8.0.