The only Google service I rely on is Gmail, because it's the only Google service I can have a reasonable expectation of still being around in five years.
With the way they keep adding proprietary extensions to it, I wouldn't be so sure if it'll actually support sending email to non-gmail clients in five years.
Google Voice is actually a safer bet as there's no doubt all sorts of legal snares they'd hit if they shut it down. Bandwidth.com runs things behind the scenes and is a competent company to do business with. They'd likely have to spin it off if they wanted to discontinue it.
