Mike Blumenthal has been tracking the growth of Google Sites assiduously. Most of this growth has been in the developing world and not the U.S. market.
Google Sites are really are meant to be primary websites, so they probably aren’t something most businesses should consider. There’s a chance they’d compete with your existing website, and I wouldn’t suggest moving to Google Sites if you’ve already got a website.
The SEO of Google Sites is poor and while there may eventually be beneficial tie-ins to Google My Business or other Google products, those don’t currently exist. Quite frankly there are much better choices on which to build your first website, such as WordPress or Squarespace.