I’ve been spending a fair amount of time over the past couple weeks designing UIs in Balsamiq Mockup and am enjoying it. Gadgetopia has a decent review that I mostly agree with. To summarize: it’s super simple, easy to use, and provides a lot of common interface elements.
The part I disagreed with:
The only drawback of Balsamiq Mockups is that you run the risk of getting a little cutesy. Everything in it is of this “sketch” style, which might get old after a while.
This is actually something I particularly like about Balsamiq – the “handwritten” informal look of the lines & letters says something about the fidelity of the mockup. I remember reading a UX blog post that resonated with me (can’t find it now, will keep looking) which recommended using overlapping lines for edges to indicate that the screen shouldn’t be taken as a final design but something needing input and refinement. The hand drawn buttons / text / edges in Balsmiq convey this pretty nicely.