Acer C120 is obviously limited. Within those limits, though, it's a potentially attractive choice. Small and light enough to throw in your laptop bag without a second thought, it's also easy to set up. One particularly nice touch is that it can get its power from the same USB connection it uses for a data image, so you can skip plugging in the power block or even lugging it around. If you choose that route, however, you'll lose some brightness compared to when you use the power block.
Like most pocket projectors, the C120 is built around a DLP chip and LED light source. The LEDs offer a 20,000 hour rated lifetime with a 1-year warranty according to Acer (even though the Web site currently says 90 days).
The 854x480 native resolution, with its roughly 16:9 aspect ratio, is reasonably common for a projector that weighs well under half a pound, but the 100 lumen brightness rating with the power block is at the high end of the range for the weight. If 100 lumens doesn't sound like a lot, keep in mind that earlier generations of pocket projectors offered ratings as low as 10 lumens and were bright enough to be useful.
The C120's key limitation is its lack of any connector for a video source. More than that, depending on your laptop, you may not even be able show video images played on a computer. This makes the C120 strictly a presentation tool for data projection. If presentations are what you need it for, however, it can do the job well enough to easily make it worth the $259 street price.