I use Balsamiq quite a lot for screen mockups for clients. Over the years I found that users get very fixated on a prototype that it too ‘polished’ as they expect the end result to be identical to that (I learned that one the hard way). I like Balsamiq as the mockups are clearly rough and can’t be mistaken for a finished product. Unlike Axure RP, it’s not really designed for fully interactive prototypes but when you just need a quick ‘n’ dirty way to communicate a mockup with the users it’s perfect for the job.

Balsamiq is a great tool and at a budget price of $79 it’s tough to beat.
www.balsamiq.com