Font Geeks tend to put all fonts in one of two categories: Free, or Good. The good ones cost money, though the rates vary. The free ones tend to be garishly over-decorative, or just rip-offs of well designed fonts. But these look-alike versions tend to not have the extended character sets of quality fonts, no ligatures (combined letter forms), no accented characters, a lack of versions (italic, bold, etc), and often very poor kerning (the spacing between letters on the page).
So finding a sophisticated, elegant font, with a full extended character set, excellent versioning, and superb kerning that is free is quite surprising. The advantages of this for those in the design profession is very clear: you, your clients, your printers, your associates, can all have a copy, without the “look the other way and loan them a copy” syndrome that tends to occur with paid fonts. For documents to move effortlessly between people in a working group, the fonts involved must be on all the systems. And only the (lets admit it, rather boring) common system fonts really fit that description. The system fonts aren’t bad, but inevitably are overused, so lack the freshness of a professional font.
So Open Sans is interesting for all the reasons above, plus a rather different reason: it is distributed under a common Open Source license. We can debate the virtues of Open Source for software development, but it makes a lot of sense for fonts, given the complexities of font distribution with a document. Another interesting tidbit is that WordPress, a major Blog and Website host, is now using Open Sans for their own default font. See an example WordPress page in Open Sans here.
You can download a TrueType version of Open Sans here, to try it yourself. Enjoy!