I use it, and there are some things I like about it. You can choose a number. My phone number spells out goEMAW, so it is easy for me to remember. I give my GV number to people I don't really want to talk to, like my credit card company and all the other services I have that require a phone number.
The voice mail feature is great. If somebody leaves me a voicemail, I get an email with a transcript of the voicemail immediatly. So I don't have to waste my time calling in and listening to some stupid message, I can just delete the thing in a second or two.
I also have an app called Groove IP that uses your GV number to make VOIP calls for free. I do this because I have a limited number of minutes on my cell phone plan. The call quality isn't great so I don't use it when I call people I like, but it works great if you are calling your credit card company or something.
I can't really think of any cons. Worst case scenario is you don't really think it is very valuable, but it is free so the only thing you lost is the 15 minutes it takes to set up.