If you can get your hands on a Google Voice account—do it. Google Voice can save you money, save you time, and you never have to give out your personal cell phone number again if you do not want to.
When you get a Google Voice account with a Google number, you get a free phone number that can be paired up with your current cell phone number. You get free visual voicemail—voicemails are organized like emails. You get free voicemail transcription—super handy for lawyers on the move. You can also require that callers identify themselves—when your phone rings and you answer you will hear their name before you choose whether to accept or reject the call. You can also easily record your phone calls and store them online in your Google Voice account (also super handy for lawyers). Lastly, you get free SMS text messaging. One major bummer, however, is that Google Voice will not work on your iPhone.
Overall—this is one great product. If you want clients to reach you on your cell phone, give them your Google Voice number and setup Google Voice to forward to your cell phone. You can also setup Google Voice to only forward Monday through Friday. You can easily call out from your cell phone with your Google Voice number by calling into your Google voicemail and pressing 2 to make a call. Another big advantage is that you can access Google Voice either using your cell phone or from a browser on your computer.
Google also just changed the program to allow Google Voice for existing numbers, minus some of the better features of Google Voice with a Google number. There is no free SMS texting, you cannot record your calls, and there is no caller identification feature. Even without those features, the visual voicemail and transcription is probably worth signing up.