The Optoma Pico PK101 is certainly fun to play with. You'll have to decide, however if you have a use that makes sense. The Pico is all about portability, tiny, lightweight, and battery powered.
If you have a portable application, be it gaming, watching YouTube videos, or just showing your friends your latest photos on your iPhone, digital camera, or other device with a composite video output, then the Pico is in its element. It also works fine with any camcorder with composite video out (most camcorders, if not all).
Picture quality is ok, but not exceptional. Resolution is basic - half of VGA, and it does have a small speaker for sound. Bottom line is that it delivers pretty much what you expect. Teenagers will have no problem with the quality, when showing friends the latest photos they took on their phones, and cameras. I can definitely see that crowd using it for YouTube as well.
It is mentioned that the Pico can be used for business presentations, and that's true. Since it only handles composite video, you need to output your Powerpoint type presentation as a series of jpgs, that can be used in a slideshow.
I do question, however, if business people will buy into the Pico. After the presenter's audience gets over the wow factor of such a small projector, they still have to deal with a small, not bright, and relatively unsharp image, compared to the least expensive business projectors. I would say that for a business application, from an ultra small projector, people would be happier with some of the brighter, micro-projectors like the Mitsubish PK series. Some of those, now, are claiming up to, or more than 100 lumens. (We'll be testing the new BenQ shortly). Most of those next sized up LED projectors are 1/2 pound plus, and many times the size, but 3 to 10 times brighter, and that's important. Also some of those have computer inputs, and most are also battery powered.
Keep in mind, this is the first generation. Future projectors like the Pico are likely to have a bit more brightness, and more inputs. (At least one is SVGA (800x600) and I believe one has a VGA port as well as video input.)
Getting back to the Optoma Pico - it certainly is cool. I can see a number of folks really into the latest gadgets, being drawn to the Pico (or one of its competitors). We'll just have to see how long it takes Optoma and other manufactures to sell millions of them.