The immediate benefit for using GV is the international calling rates.
I was paying Sprint $4.00 a month for "discounted" international calling. I call Hong Kong regularly and was paying close to $0.10 a minute. With GV...I pay $0.02 a minute with no start up cost. Even if it burns into my "landline" minutes...I still end up saving money in the long run.
Also, with the transcription of voicemail by GV...it is NOT ALWAYS required to listen to a voicemail. Even though the transcription is not very accurate right now...thru context...I can tell the gist of a voicemail. Actually its pretty funny seeing what GV comes up with when my grandma leaves a message in Chinese!!!