Open Sans like all the other Helvetica replacements (except Arial) are only cool the first few days. After that it's like, nah, gimme back Helvetica (which have been cool since the 70s):
Helvetica is indeed cool (the Neue family cooler, IMO) and has been since the 50s, and _Helvetica_ is also cool. Thinking of Helvetica as the only worthwhile sans-serif typeface is less cool :)
font-family: 'Helvetica Neue',Arial,sans-serif; font-family: Arial, sans-serif; font-family: sans-serif;
For pixel-perfect alignment, you need to deal with some minor size differences between the fonts, but pixel-perfect design is not really achievable in many designs anyway.
I’m more than happy to pay Typekit to provide my website visitors with high quality fonts that are optimized for all major platforms and conform to my clients’ brand guidelines. Adobe Typekit[1] even has a free tier that lets you use many of the fonts in its catalog and all requests are handled by their CDNs, saving you bandwidth (=money).
Even when I have carte blanche, I rather use Myriad or Frutiger over Open Sans. On OS X and iOS, Open Sans appears unbalanced to me.
As for the openness: if somehow Typekit’s CDNs are down, there’s always the fallback fonts, like Helvetica/Arial. I don’t see a need to start hosting fonts myself or settle for less than ideal fonts hosted elsewhere.
[1] I subscribed to Typekit years before it was acquired by Adobe, and I feared Adobe would ruin it, but to my surprise, since the acquisition the service has only gotten better.