As a few other have said, I find them extremely helpul in keeping the focus on the elements of the wireframe instead of the look and feel. I also find it helps encourage a feeling of a "work in progress" and encourages more discussion. It's the equivalent of drawing on a notepad or a whiteboard to show placement of elements but make it clear that it's only a guide for final design, not what it will actually look like. The "unfinished" aspect seems to encourage more collaboration and discussion, because by it's very design you can see that it's not "done done".

They don't look like they were drawn by a little kid - if that's the case, the problem is wth your mockup, not the tool or style of the lines.

Whether or not you deliver the sketchy version to your client is up to you, your client, the project, and their expectations. If you have a client that enjoys being more hands-on (and is self-aware enough to know where they can be an asset in the process), collaboration can be great. Our design department gets a functional, sketchy prototype from which to start designing, which helps them remember that their design shouldnt be to heavily influenced by the look and feel of the mockup.

Sketchy wireframes may not be appropriate as a deliverable for every client on every project, but they are a staple part of our production process now, and everyone prefers them over straight-lines for the beginning phases of wireframing where feedback and collaboraton is a goal. Some tools, like iPlotz, allow you switch back and forth from sketchy to straight lines, so moving them out of the "rough" phase and into "much closer to final" or "final" phase doesn't require completely re-working the entire thing, just the click of a button.

There's a discussion on it more here: http://www.boxesandarrows.com/view/sketchy-wireframes

I don't like or dislike having sketchy looking UIs :-)

Depending on the context though I do occasionally find them useful.

A certain kind of client can view a functional prototype as being close to "finished". Using a visual appearance that is obviously sketchy helps prevent those kind of incorrect assumptions.

I think the idea is to prevent people from focusing on the visual design of the wireframes by making it clear that they are sketches, not final designs.

This works with paper sketches, ensuring people pay attention to layout, function and content, but I'm not sure about when it's digitised. On-screen the sketchy style is so unnatural that it presents a definitive style in itself, which detracts from exactly what they were trying to achieve in the first place!

Seems kinda pointless to me, I think standard straight lines do the job perfectly well.

From http://balsamiq.com/products/mockups

DESIGNED FOR COLLABORATION... a look no-one is afraid to criticize: wireframes created with Mockups are intentionally rough and low-fidelity. The idea is to encourage as much feedback as possible...no-one will think you'll be offended by their input, they'll know immediately that you just 'threw it together' quickly.

I agree with the idea. It makes other parties feel comfortable with suggesting what they need to from their point of view.

Like Pete, I think that trying to imitate paper sketching by using sketchy looking lines and the like is not the right way to convey the non-finality of a design on screen. Other than on paper it feels quite artificial and hence for me rather gets constantly in the way instead of staying in the background. The issue is not easily solved though – simple straight lines etc. due to their precision don't look necessarily this different from something final.