Google Voice is a free telecom service which provides you with a telephone number. Using Google's web app, you can redirect calls made to this number to any other number (such as your cell, home, or office number), or ring multiple numbers simultaneously. If someone leaves you a voicemail message using your Google Voice number, Google can transcribe it and email or text it to you. Text messages to your Google Voice number can be emailed as well, and they're free. You can use the service to place calls over the internet, listen to your voicemail online, set up number-specific voicemail greetings, route calls from certain numbers to certain phones, place conference calls, block/screen callers, forward/download voicemail messages, record/archive calls online, and place low-rate international calls.
As for me, Google Voice has been primarily beneficial for my wife Erin's small business. Google Voice provides her a free, local work phone number which forwards to her cell phone. It just makes sense.
However, the benefits fall off sharply there. Erin has an Android smartphone, so placing calls from her Google Voice number (ensuring that when she calls someone, her Google Voice number will show up in their caller ID) is easy. However, if you have any other phone or smartphone, you must call your Google Voice number first and then punch in the number you're calling to achieve this. Or alternately, you can add your Google Voice number to the beginning of all the contact numbers in your cell phone.
In addition, because calls using your Google Voice number are routed through Google's system, you do not get in-network calling benefits if you use Google Voice. So for instance, if I call someone who is on Verizon from my Verizon cell phone, but I use Google Voice, Verizon will charge me as if I was calling a non-Verizon number.
The voicemail transcribing feature would be a huge benefit if it worked well. Unfortunately the transcriptions are hardly ever comprehensible. The most use I've gotten out of them is reading them back to the caller later for a laugh. Having my voicemail messages archived online seemed like a great idea at first too, but once I tried it I realized most voicemail messages are temporal; I listen to them quickly, respond, and then I don't need them anymore.
The biggest detriment to the service however is delays. Erin and I have both experienced delays up to 24 hours in receiving voicemail messages and texts made to our Google Voice number, and when using Google Voice to handle voicemail messages left to our primary cell phone numbers (another feature of the service). However this only seems to apply to cell phone notifications; Erin has never experienced a delay in email notification of a text or voicemail message. She is able to use the service by relying on her phone's email alerts to notify her of new voicemail messages.
As for me, I've simply resorted to using my Google Voice number only when I need to give someone (such as a mechanic or someone with a home phone) a local number, since my primary cell phone number still has an out-of-state area code. This, along with the advantages for small businesses, seem to me to be the best use of the service. Free text messages and voicemail transcribing are great, but only if they're on time. Reliability is key.
Update 6/20/11: I'm told Blackberry also has a Google Voice app which allows you to easily make calls from your Google Voice number.