yikes... saw it and i would not recommend it at all.
why?

mainly cause they are re-creating existing websites/applications. so the 'design' is already figured out and all you're doing is re-creating it using pre-drawn items. like 'coverflow.' come on... if you didn't already see it being used on iTunes and they didn't have it as a default drawn item would you have known to draw that into the app?

and the fact 'coverflow' is patented by Apple - what good is it having it as a default pre-drawn item?

when i design apps / websites for clients i use 'real' content. that means content that could be real - not fake content using real UI elements. the problem with having a 'pencil' drawing is that it doesn't give you a good representation of what the app / website will look like. i've had many many instances where a PM will come to me with Visio mockups - which when translated looks like crap or just doesn't work.

this MAY be a good tool for someone who actually understands what the design limitations of a app / website is but for those who have little experience it's going to give them bad results - which in the 'mockup' looks great - but then will look horrible when implemented.

I think wireframing can be a great way to get requirements down -- as long as there is an understanding that a professional designer may move things around to make the design work with real data.

Reread you comments and also agree with your last point. Such a tool can be dangerous if someone doesn't understand design limitation.

Something funny to look at as well, their entire template and style for their promo page is an amazing rip-off of the 37signals pages (check out Basecamp here: http://www.basecamphq.com/?). I kind of think the product is interesting but I think it would have shown better if they had used their own layout and shown a little creativity (at least not outright theft). I also really like how the guy makes some if his own sound effects...

Read my blog post for why I like it (though I'll admit I haven't bought it yet -- I've just used the web version). I have not used a single one of the things like the Cover Flow element and I agree that they're pretty useless. The only exception is the browser element where I wanted to show a mockup inside a browser (to emphasize that's where it was). Most of the mockups I've done were composed of just text, rectangles, lines, buttons, etc., and, for doing things like this, it was actually faster than doing it on paper and, for me, that is what it is replacing.

If I'm drawing something that goes beyond that, Balsamiq Mockups isn't going to fly -- and, in the last month, I have also used both Photoshop and Illustrator for different mockups for just that reason.

Example 1: Account & Billing UI, Forms, etc - perfect for Balsamiq Mockups (bear in mind this UI is wrapped in standard site UI)

Example 2: Custom visitor UI for a web site - Photoshop or Illustrator

Note: I also agree with Sung's last point. Handing it (or a pencil and paper) to someone who doesn't actually understand design will not result in a good design.