I love the idea of Balsamiq Mockups; it's one of those idea which makes you wonder why it took someone so long to come up with it. Knock together a few UI mockups from common UI elements and you've got a great way to preview sites and programs to your client.
In fact I've seen people use the Visual Studio win/web forms drag and drop for this sort of thing but it's pretty limited in terms of the UI elements you can use and the amount of information you can present without doing some coding work. When Balsamiq Mockups lets you put a data table on the page, for example, you can do headers, links and data with a simple comma separated list. It lets you display a placeholder for a mapping interface with absolutely no code.
So the idea and the basic implementation are both great, but I'm not sold on the larger picture. If you're trying to explore a UI then you're going to need more than one screen. It looks like Balsamiq Mockups supports this on the desktop version by letting you save mockups and load them later, but I'd really like to see a way of having multiple mockups together in one canvas. I think there's a case for saying that this increases the complexity of BM in that many would like to see a way of showing how each mockup links to the next. I'm not too bothered about that, I'd just like to see multiple "pages", each presenting a different mockup.
The other thing that baffles me is the ability to get an online preview of BM but not being able to use the software online once you've bought it. I'd really love to see an edition where I can use the software online, save my mockups in my online account, and be able to access them from anywhere. I'm writing this from my netbook and it'd be great to look at the mockups I'd created on my desktop without having to pass the files over.
I don't want these points to distract from Balsamiq Mockups though, because it's great, a revelation in fact. The way it allows us to piece together UI ideas without resorting to pen and paper or a paint package is a dream come true and gives us another method of rapidly prototyping for our end users.